html elementSupport in all current engines.
Support in all current engines.
head element followed by a body element.[Exposed=Window]
interface HTMLHtmlElement : HTMLElement {
[HTMLConstructor] constructor();
// also has obsolete members
};The html element represents the root of an HTML document.
Authors are encouraged to specify a lang attribute on the root
html element, giving the document's language. This aids speech synthesis tools to
determine what pronunciations to use, translation tools to determine what rules to use, and so
forth.
The html element in the following example declares that the document's language
is English.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Swapping Songs</title>
</head>
<body>
<h1>Swapping Songs</h1>
<p>Tonight I swapped some of the songs I wrote with some friends, who
gave me some of the songs they wrote. I love sharing my music.</p>
</body>
</html>head elementSupport in all current engines.
Support in all current engines.
html element.iframe srcdoc document or if title information is available from a higher-level protocol: Zero or more elements of metadata content, of which no more than one is a title element and no more than one is a base element.title element and no more than one is a base element.[Exposed=Window]
interface HTMLHeadElement : HTMLElement {
[HTMLConstructor] constructor();
};The head element represents a collection of metadata for the
Document.
The collection of metadata in a head element can be large or small. Here is an
example of a very short one:
<!doctype html>
<html lang=en>
<head>
<title>A document with a short head</title>
</head>
<body>
...Here is an example of a longer one:
<!DOCTYPE HTML>
<HTML LANG="EN">
<HEAD>
<META CHARSET="UTF-8">
<BASE HREF="https://www.example.com/">
<TITLE>An application with a long head</TITLE>
<LINK REL="STYLESHEET" HREF="default.css">
<LINK REL="STYLESHEET ALTERNATE" HREF="big.css" TITLE="Big Text">
<SCRIPT SRC="support.js"></SCRIPT>
<META NAME="APPLICATION-NAME" CONTENT="Long headed application">
</HEAD>
<BODY>
...The title element is a required child in most situations, but when a
higher-level protocol provides title information, e.g. in the Subject line of an email when HTML
is used as an email authoring format, the title element can be omitted.
title elementSupport in all current engines.
Support in all current engines.
head element containing no other title elements.[Exposed=Window]
interface HTMLTitleElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString text;
};The title element represents the document's title or name. Authors
should use titles that identify their documents even when they are used out of context, for
example in a user's history or bookmarks, or in search results. The document's title is often
different from its first heading, since the first heading does not have to stand alone when taken
out of context.
There must be no more than one title element per document.
If it's reasonable for the Document to have no title, then the
title element is probably not required. See the head element's content
model for a description of when the element is required.
title.text [ = value ]Returns the child text content of the element.
Can be set, to replace the element's children with the given value.
The text
attribute's getter must return this title element's child text
content.
The text attribute's setter must string replace
all with the given value within this title element.
Here are some examples of appropriate titles, contrasted with the top-level headings that might be used on those same pages.
<title>Introduction to The Mating Rituals of Bees</title>
...
<h1>Introduction</h1>
<p>This companion guide to the highly successful
<cite>Introduction to Medieval Bee-Keeping</cite> book is...The next page might be a part of the same site. Note how the title describes the subject matter unambiguously, while the first heading assumes the reader knows what the context is and therefore won't wonder if the dances are Salsa or Waltz:
<title>Dances used during bee mating rituals</title>
...
<h1>The Dances</h1>The string to use as the document's title is given by the document.title IDL attribute.
User agents should use the document's title when referring to the document in their user
interface. When the contents of a title element are used in this way, the
directionality of that title element should be used to set the directionality
of the document's title in the user interface.
base elementSupport in all current engines.
Support in all current engines.
head element containing no other base elements.hreftarget[Exposed=Window]
interface HTMLBaseElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString href;
[CEReactions] attribute DOMString target;
};The base element allows authors to specify the document base URL for
the purposes of parsing URLs, and the name of the default
browsing context for the purposes of following hyperlinks. The element
does not represent any content beyond this information.
There must be no more than one base element per document.
A base element must have either an href
attribute, a target attribute, or both.
The href content
attribute, if specified, must contain a valid URL potentially surrounded by
spaces.
A base element, if it has an href attribute,
must come before any other elements in the tree that have attributes defined as taking URLs, except the html element (its manifest attribute isn't affected by base
elements).
If there are multiple base elements with href attributes, all but the first are ignored.
The target attribute,
if specified, must contain a valid browsing context name or keyword, which specifies
which browsing context is to be used as the default when hyperlinks and forms in the
Document cause navigation.
A base element, if it has a target
attribute, must come before any elements in the tree that represent hyperlinks.
If there are multiple base elements with target attributes, all but the first are ignored.
To get an element's target, given an a, area, or
form element element, run these steps:
If element has a target attribute, then return that
attribute's value.
If element's node document contains a base element
with a target attribute, then return the value of the
target attribute of the first such base
element.
Return the empty string.
A base element that is the first base element with an href content attribute in a document tree has a
frozen base URL. The frozen base URL must be immediately
set for an element whenever any of the following
situations occur:
base element becomes the first base element in tree
order with an href content attribute in its
Document.base element is the first base element in tree
order with an href content attribute in its
Document, and its href content attribute is
changed.To set the frozen base URL for an element element:
Let document be element's node document.
Let urlRecord be the result of parsing the
value of element's href content attribute with
document's fallback base URL, and document's character encoding. (Thus, the base
element isn't affected by itself.)
Set element's frozen base URL to document's
fallback base URL, if urlRecord is failure or running Is base
allowed for Document? on the resulting URL record and document
returns "Blocked", and to urlRecord otherwise.
The href IDL
attribute, on getting, must return the result of running the following algorithm:
Let document be element's node document.
Let url be the value of the href
attribute of this element, if it has one, and the empty string otherwise.
Let urlRecord be the result of parsing
url with document's fallback base URL, and
document's character encoding.
(Thus, the base element isn't affected by other base elements or
itself.)
If urlRecord is failure, return url.
Return the serialization of urlRecord.
The href IDL attribute, on setting, must set the href content attribute to the given new value.
The target IDL
attribute must reflect the content attribute of the same name.
In this example, a base element is used to set the document base
URL:
<!DOCTYPE html>
<html lang="en">
<head>
<title>This is an example for the <base> element</title>
<base href="https://www.example.com/news/index.html">
</head>
<body>
<p>Visit the <a href="archives.html">archives</a>.</p>
</body>
</html>The link in the above example would be a link to "https://www.example.com/news/archives.html".
link elementSupport in all current engines.
Support in all current engines.
noscript element that is a child of a head element.hrefcrossoriginrelmediaintegrityhreflangtypereferrerpolicysizesimagesrcsetimagesizesascolordisabledtitle attribute has special semantics on this element.[Exposed=Window]
interface HTMLLinkElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute USVString href;
[CEReactions] attribute DOMString? crossOrigin;
[CEReactions] attribute DOMString rel;
[CEReactions] attribute DOMString as; // (default "")
[SameObject, PutForwards=value] readonly attribute DOMTokenList relList;
[CEReactions] attribute DOMString media;
[CEReactions] attribute DOMString integrity;
[CEReactions] attribute DOMString hreflang;
[CEReactions] attribute DOMString type;
[SameObject, PutForwards=value] readonly attribute DOMTokenList sizes;
[CEReactions] attribute USVString imageSrcset;
[CEReactions] attribute DOMString imageSizes;
[CEReactions] attribute DOMString referrerPolicy;
[CEReactions] attribute boolean disabled;
// also has obsolete members
};
HTMLLinkElement includes LinkStyle;The link element allows authors to link their document to other resources.
The address of the link(s) is given by the href attribute. If the href attribute is present, then its value must be a valid
non-empty URL potentially surrounded by spaces. One or both of the href or imagesrcset
attributes must be present.
If both the href and imagesrcset attributes are absent, then the element does not
define a link.
The types of link indicated (the relationships) are given by the value of the rel attribute, which, if present, must have a
value that is a unordered set of unique space-separated tokens. The allowed keywords and their meanings are defined in a later section. If the rel attribute is absent, has no keywords, or if
none of the keywords used are allowed according to the definitions in this specification, then the
element does not create any links.
rel's
supported tokens are the keywords defined in
HTML link types which are allowed on link elements, impact
the processing model, and are supported by the user agent. The possible supported tokens are
alternate,
dns-prefetch,
icon,
manifest,
modulepreload,
next,
pingback,
preconnect,
prefetch,
preload,
prerender,
search, and
stylesheet.
rel's supported
tokens must only include the tokens from this list that the user agent implements the
processing model for.
Theoretically a user agent could support the processing model for the canonical keyword — if it were a search engine that executed
JavaScript. But in practice that's quite unlikely. So in most cases, canonical ought not be included in rel's supported
tokens.
A link element must have either a rel
attribute or an itemprop attribute, but not both.
If a link element has an itemprop attribute,
or has a rel attribute that contains only keywords that are
body-ok, then the element is said to be allowed in the body. This means
that the element can be used where phrasing content is expected.
If the rel attribute is used, the element can
only sometimes be used in the body of the page. When used with the itemprop attribute, the element can be used both in the
head element and in the body of the page, subject to the constraints of
the microdata model.
Two categories of links can be created using the link element: links to external resources and hyperlinks. The link types section defines
whether a particular link type is an external resource or a hyperlink. One link
element can create multiple links (of which some might be external resource links and some might be hyperlinks); exactly which and how many links are created depends on the
keywords given in the rel attribute. User agents must process
the links on a per-link basis, not a per-element basis.
Each link created for a link element is handled separately. For
instance, if there are two link elements with rel="stylesheet",
they each count as a separate external resource, and each is affected by its own attributes
independently. Similarly, if a single link element has a rel attribute with the value next stylesheet,
it creates both a hyperlink (for the next keyword) and
an external resource link (for the stylesheet
keyword), and they are affected by other attributes (such as media or title)
differently.
For example, the following link element creates two hyperlinks (to the same page):
<link rel="author license" href="/about">The two links created by this element are one whose semantic is that the target page has information about the current page's author, and one whose semantic is that the target page has information regarding the license under which the current page is provided.
Hyperlinks created with the link element and its
rel attribute apply to the whole document. This contrasts with
the rel attribute of a and area
elements, which indicates the type of a link whose context is given by the link's location within
the document.
Unlike those created by a and area elements, hyperlinks created by link elements are not displayed as
part of the document by default, in user agents that support the suggested
default rendering. And even if they are force-displayed using CSS, they have no
activation behavior. Instead, they primarily provide semantic information which might
be used by the page or by other software that consumes the page's contents. Additionally, the user
agent can provide
its own UI for following such hyperlinks.
The exact behavior for links to external resources depends on the exact relationship, as defined for the relevant link type.
The crossorigin
attribute is a CORS settings attribute. It is intended for use with external resource links.
The media attribute
says which media the resource applies to. The value must be a valid media query
list.
The integrity
attribute represents the integrity
metadata for requests which this element is responsible for. The value is text. The
attribute must only be specified on link elements that have a rel attribute that contains the stylesheet, preload, or modulepreload keyword. [SRI]
The hreflang
attribute on the link element has the same semantics as the hreflang attribute on the a
element.
The type attribute
gives the MIME type of the linked resource. It is purely advisory. The value must be
a valid MIME type string.
For external resource links, the type attribute is used as a hint to user agents so that they can
avoid fetching resources they do not support.
The referrerpolicy attribute is a referrer policy
attribute. It is intended for use with external
resource links, where it helps set the referrer policy used when fetching and processing the linked resource.
[REFERRERPOLICY].
The title attribute
gives the title of the link. With one exception, it is purely advisory. The value is text. The
exception is for style sheet links that are in a document tree, for which the title attribute defines CSS
style sheet sets.
The title attribute on link
elements differs from the global title attribute of most other
elements in that a link without a title does not inherit the title of the parent element: it
merely has no title.
The imagesrcset
attribute may be present, and is a srcset attribute.
The imagesrcset and href attributes (if width
descriptors are not used) together contribute the image
sources to the source set.
If the imagesrcset attribute is present and has any
image candidate strings using a width
descriptor, the imagesizes attribute must also be present, and is a
sizes attribute. The imagesizes attribute
contributes the source size to the source set.
The imagesrcset and imagesizes attributes must only be specified on
link elements that have both a rel attribute that
specifies the preload keyword, as well as an as attribute in the "image" state.
These attributes allow preloading the appropriate resource that is later used by an
img element that has the corresponding values for its srcset and sizes
attributes:
<link rel="preload" as="image"
imagesrcset="wolf_400px.jpg 400w, wolf_800px.jpg 800w, wolf_1600px.jpg 1600w"
imagesizes="50vw">
<!-- ... later, or perhaps inserted dynamically ... -->
<img src="wolf.jpg" alt="A rad wolf"
srcset="wolf_400px.jpg 400w, wolf_800px.jpg 800w, wolf_1600px.jpg 1600w"
sizes="50vw">Note how we omit the href attribute, as it would only
be relevant for browsers that do not support imagesrcset, and in those cases it would likely cause the
incorrect image to be preloaded.
The imagesrcset attribute can be combined with the
media attribute to preload the appropriate resource
selected from a picture element's sources, for art direction:
<link rel="preload" as="image"
imagesrcset="dog-cropped-1x.jpg, dog-cropped-2x.jpg 2x"
media="(max-width: 800px)">
<link rel="preload" as="image"
imagesrcset="dog-wide-1x.jpg, dog-wide-2x.jpg 2x"
media="(min-width: 801px)">
<!-- ... later, or perhaps inserted dynamically ... -->
<picture>
<source srcset="dog-cropped-1x.jpg, dog-cropped-2x.jpg 2x"
media="(max-width: 800px)">
<img src="dog-wide-1x.jpg" srcset="dog-wide-2x.jpg 2x"
alt="An awesome dog">
</picture>The sizes attribute
gives the sizes of icons for visual media. Its value, if present, is merely advisory. User agents may use the value to decide which icon(s) to use if multiple icons are
available. If specified, the attribute must have a value that is an unordered set of
unique space-separated tokens which are ASCII case-insensitive. Each value
must be either an ASCII case-insensitive match for the string "any", or a value that consists of two valid non-negative integers that do not have a leading U+0030 DIGIT
ZERO (0) character and that are separated by a single U+0078 LATIN SMALL LETTER X or U+0058 LATIN
CAPITAL LETTER X character. The attribute must only be specified on link elements
that have a rel attribute that specifies the icon keyword or the apple-touch-icon keyword.
The apple-touch-icon keyword is a registered extension to the predefined set of link types, but user
agents are not required to support it in any way.
The as attribute
specifies the potential destination for a
preload request for the resource given by the href attribute.
It is an enumerated attribute. Each potential destination is a keyword for this
attribute, mapping to a state of the same name. The attribute must be specified on
link elements that have a rel attribute that
contains the preload keyword. It may be specified on
link elements that have a rel attribute that
contains the modulepreload keyword; in such cases it must
have a value which is a script-like
destination. For other link elements, it must not be specified.
The processing model for how the as attribute is
used is given in an individual link type's fetch and process the linked resource
algorithm.
The attribute does not have a missing value
default or invalid value default, meaning that invalid
or missing values for the attribute map to no state. This is accounted for in the processing
model. For preload links, both conditions are an error; for
modulepreload links, a missing value will be treated as
"script".
The color attribute is
used with the mask-icon link type. The attribute must only be specified on
link elements that have a rel attribute that
contains the mask-icon keyword. The value must be a string that matches the
CSS <color> production, defining a suggested color that user agents can use to
customize the display of the icon that the user sees when they pin your site.
This specification does not have any user agent requirements for the color attribute.
The mask-icon keyword is a registered extension to the predefined set of link types, but user
agents are not required to support it in any way.
link elements have an associated explicitly enabled boolean. It is
initially false.
The disabled
attribute is a boolean attribute that is used with the stylesheet link type. The attribute must only be specified on
link elements that have a rel attribute that
contains the stylesheet keyword.
Whenever the disabled attribute is removed, set the
link element's explicitly enabled attribute to true.
Removing the disabled attribute dynamically, e.g.,
using document.querySelector("link").removeAttribute("disabled"), will
fetch and apply the style sheet:
<link disabled rel="alternate stylesheet" href="css/pooh">Support in all current engines.
The IDL attributes
href,
hreflang,
integrity,
media,
rel,
sizes,
type, and
disabled
each must reflect the respective content attributes of the same name.
There is no reflecting IDL attribute for the color attribute, but this might be added later.
Support in all current engines.
The as IDL
attribute must reflect the as content attribute,
limited to only known values.
The crossOrigin IDL attribute must reflect the
crossorigin content attribute, limited to only
known values.
HTMLLinkElement/referrerPolicy
Support in all current engines.
The referrerPolicy IDL attribute must
reflect the referrerpolicy content
attribute, limited to only known values.
The imageSrcset IDL attribute must reflect the
imagesrcset content attribute.
The imageSizes IDL attribute must reflect the
imagesizes content attribute.
Support in all current engines.
The relList
IDL attribute must reflect the rel content attribute.
media attributeIf the link is a hyperlink then the media
attribute is purely advisory, and describes for which media the document in question was
designed.
However, if the link is an external resource link, then the media attribute is prescriptive. The user agent must apply the
external resource when the media attribute's value
matches the environment and the other relevant conditions apply, and must not apply
it otherwise.
The default, if the media attribute is
omitted, is "all", meaning that by default links apply to all media.
The external resource might have further restrictions defined within that limit
its applicability. For example, a CSS style sheet might have some @media
blocks. This specification does not override such further restrictions or requirements.
type attributeIf the type attribute is present, then the user agent must
assume that the resource is of the given type (even if that is not a valid MIME type
string, e.g. the empty string). If the attribute is omitted, but the external
resource link type has a default type defined, then the user agent must assume that the
resource is of that type. If the UA does not support the given MIME type for the
given link relationship, then the UA should not fetch and process the linked
resource; if the UA does support the given MIME type for the given link
relationship, then the UA should fetch and process the linked resource at the
appropriate time as specified for the external resource link's particular type.
If the attribute is omitted, and the external resource link type does not have a
default type defined, but the user agent would fetch and process the linked resource
if the type was known and supported, then the user agent should fetch and process the linked
resource under the assumption that it will be supported.
User agents must not consider the type attribute
authoritative — upon fetching the resource, user agents must not use the type attribute to determine its actual type. Only the actual type
(as defined in the next paragraph) is used to determine whether to apply the resource,
not the aforementioned assumed type.
If the external resource link type defines rules for processing the resource's Content-Type metadata, then those rules apply. Otherwise, if the resource is expected to be an image, user agents may apply the image sniffing rules, with the official type being the type determined from the resource's Content-Type metadata, and use the resulting computed type of the resource as if it was the actual type. Otherwise, if neither of these conditions apply or if the user agent opts not to apply the image sniffing rules, then the user agent must use the resource's Content-Type metadata to determine the type of the resource. If there is no type metadata, but the external resource link type has a default type defined, then the user agent must assume that the resource is of that type.
The stylesheet link type defines rules for
processing the resource's Content-Type metadata.
Once the user agent has established the type of the resource, the user agent must apply the resource if it is of a supported type and the other relevant conditions apply, and must ignore the resource otherwise.
If a document contains style sheet links labeled as follows:
<link rel="stylesheet" href="A" type="text/plain">
<link rel="stylesheet" href="B" type="text/css">
<link rel="stylesheet" href="C">...then a compliant UA that supported only CSS style sheets would fetch the B and C files, and
skip the A file (since text/plain is not the MIME type for CSS style
sheets).
For files B and C, it would then check the actual types returned by the server. For those that
are sent as text/css, it would apply the styles, but for those labeled as
text/plain, or any other type, it would not.
If one of the two files was returned without a Content-Type metadata, or with a
syntactically incorrect type like Content-Type: "null", then the
default type for stylesheet links would kick in. Since that
default type is text/css, the style sheet would nonetheless be applied.
link elementAll external resource
links have a fetch and process the linked resource algorithm, which takes a
link element el. They also have linked resource fetch setup
steps which take a link element el and request request. Individual link types may provide
their own fetch and process the linked resource algorithm, but unless explicitly
stated, they use the default fetch and process the linked resource algorithm.
Similarly, individual link types may provide their own linked resource fetch setup
steps, but unless explicitly stated, these steps just return true.
The default fetch and process the linked resource, given a link element
el, is as follows:
If el's href attribute's value is the
empty string, then return.
Parse a URL given el's href
attribute, relative to el's node document. If that fails, then return.
Otherwise, let url be the resulting URL record.
Let corsAttributeState be the current state of el's crossorigin content attribute.
Let request be the result of creating a potential-CORS request given url, the empty string, and corsAttributeState.
Set request's synchronous flag.
Set request's client to el's node document's relevant settings object.
Set request's cryptographic nonce metadata to the current value of el's [[CryptographicNonce]] internal slot.
Set request's integrity
metadata to the current value of el's integrity content attribute.
Set request's
referrer policy to the current state of
el's referrerpolicy attribute.
Run the linked resource fetch setup steps, given el and request. If the result is false, then return.
Run the following steps in parallel:
Let response be the result of fetching request.
Let success be true.
If response is a network error or its status is not an ok status, set success to false.
Note that content-specific errors, e.g., CSS parse errors or PNG decoding errors, do not affect success.
If success is true, wait for the link resource's critical subresources to finish loading.
The specification that defines a link type's critical subresources (e.g., CSS) is expected to describe how these subresources are fetched and processed. However, since this is not currently explicit, this specification describes waiting for a link resource's critical subresources to be fetched and processed, with the expectation that this will be done correctly.
User agents may opt to only try to fetch and process such resources when they are needed, instead of pro-actively fetching all the external resources that are not applied.
Similar to the fetch and process the linked resource algorithm, all external resource links have a process the linked
resource algorithm which takes a link element el, boolean
success, and response response.
Individual link types may provide their own process the linked resource algorithm,
but unless explicitly stated, that algorithm does nothing.
Unless otherwise specified for a given rel keyword, the
element must delay the load event of the element's node document until
all the attempts to fetch and process the linked resource and its critical
subresources are complete. (Resources that the user agent has not yet attempted to fetch
and process, e.g., because it is waiting for the resource to be needed, do not delay the
load event.)
Link` headersHTTP `Link` headers, if supported, must be assumed to come
before any links in the document, in the order that they were given in the HTTP message. These
headers are to be processed according to the rules given in the relevant specifications. [HTTP] [WEBLINK]
Registration of relation types in HTTP `Link`
headers is distinct from HTML link types, and thus their semantics can be
different from same-named HTML types.
The processing of `Link` headers, in particular
their influence on a Document's script-blocking style sheet counter, is
not defined. See issue #4224 for
discussion on integrating this into the spec.
link
elementInteractive user agents may provide users with a means to follow the hyperlinks created using the link element, somewhere
within their user interface. The exact interface is not defined by this specification, but it
could include the following information (obtained from the element's attributes, again as defined
below), in some form or another (possibly simplified), for each hyperlink created
with each link element in the document:
rel attribute)title
attribute).href
attribute).hreflang
attribute).media
attribute).User agents could also include other information, such as the type of the resource (as given by
the type attribute).
meta
elementSupport in all current engines.
Support in all current engines.
itemprop attribute is present: flow content.itemprop attribute is present: phrasing content.charset attribute is present, or if the element's http-equiv attribute is in the Encoding declaration state: in a head element.http-equiv attribute is present but not in the Encoding declaration state: in a head element.http-equiv attribute is present but not in the Encoding declaration state: in a noscript element that is a child of a head element.name attribute is present: where metadata content is expected.itemprop attribute is present: where metadata content is expected.itemprop attribute is present: where phrasing content is expected.namehttp-equivcontentcharsetmedia[Exposed=Window]
interface HTMLMetaElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString name;
[CEReactions] attribute DOMString httpEquiv;
[CEReactions] attribute DOMString content;
[CEReactions] attribute DOMString media;
// also has obsolete members
};The meta element represents various kinds of metadata that cannot be
expressed using the title, base, link, style,
and script elements.
The meta element can represent document-level metadata with the name attribute, pragma directives with the http-equiv attribute, and the file's character encoding
declaration when an HTML document is serialized to string form (e.g. for transmission over
the network or for disk storage) with the charset
attribute.
Exactly one of the name, http-equiv, charset,
and itemprop attributes must be specified.
If either name, http-equiv, or itemprop is
specified, then the content attribute must also be
specified. Otherwise, it must be omitted.
The charset
attribute specifies the character encoding used by the document.
This is a character encoding declaration. If the attribute is present, its value must
be an ASCII case-insensitive match for the string "utf-8".
The charset attribute on the
meta element has no effect in XML documents, but is allowed in XML documents in order
to facilitate migration to and from XML.
There must not be more than one meta element with a charset attribute per document.
The content
attribute gives the value of the document metadata or pragma directive when the element is used
for those purposes. The allowed values depend on the exact context, as described in subsequent
sections of this specification.
If a meta element has a name attribute, it sets document metadata. Document metadata
is expressed in terms of name-value pairs, the name attribute
on the meta element giving the name, and the content attribute on the same element giving the value. The name
specifies what aspect of metadata is being set; valid names and the meaning of their values are
described in the following sections. If a meta element has no content attribute, then the value part of the metadata
name-value pair is the empty string.
The media attribute
says which media the metadata applies to. The value must be a valid media query list.
Unless the name is theme-color, the media
attribute has no effect on the processing model and must not be used by authors.
The name, content, and media IDL attributes
must reflect the respective content attributes of the same name. The IDL attribute
httpEquiv must
reflect the content attribute http-equiv.
Support in all current engines.
This specification defines a few names for the name
attribute of the meta element.
Names are case-insensitive, and must be compared in an ASCII case-insensitive manner.
application-nameThe value must be a short free-form string giving the name of the web application that the
page represents. If the page is not a web application, the application-name metadata name must not be used.
Translations of the web application's name may be given, using the lang attribute to specify the language of each name.
There must not be more than one meta element with a given language
and where the name attribute value is an
ASCII case-insensitive match for
application-name per document.
User agents may use the application name in UI in preference to the page's
title, since the title might include status messages and the like relevant to the
status of the page at a particular moment in time instead of just being the name of the
application.
To find the application name to use given an ordered list of languages (e.g. British English, American English, and English), user agents must run the following steps:
Let languages be the list of languages.
Let default language be the language of the
Document's document element, if any, and if that language is not
unknown.
If there is a default language, and if it is not the same language as any of the languages in languages, append it to languages.
Let winning language be the first language in languages for which
there is a meta element in the Document where the
name attribute value is an
ASCII case-insensitive match for
application-name and whose
language is the language in question.
If none of the languages have such a meta element, then return;
there's no given application name.
Return the value of the content attribute of the
first meta element in the Document in tree order where the
name attribute value is an
ASCII case-insensitive match for application-name
and whose language is winning language.
This algorithm would be used by a browser when it needs a name for the page, for instance, to label a bookmark. The languages it would provide to the algorithm would be the user's preferred languages.
authorThe value must be a free-form string giving the name of one of the page's authors.
descriptionThe value must be a free-form string that describes the page. The value must be
appropriate for use in a directory of pages, e.g. in a search engine. There must not be more than
one meta element where the name attribute value
is an ASCII case-insensitive match for
description per document.
generatorThe value must be a free-form string that identifies one of the software packages used to generate the document. This value must not be used on pages whose markup is not generated by software, e.g. pages whose markup was written by a user in a text editor.
Here is what a tool called "Frontweaver" could include in its output, in the page's
head element, to identify itself as the tool used to generate the page:
<meta name=generator content="Frontweaver 8.2">keywordsThe value must be a set of comma-separated tokens, each of which is a keyword relevant to the page.
This page about typefaces on British motorways uses a meta element to specify
some keywords that users might use to look for the page:
<!DOCTYPE HTML>
<html lang="en-GB">
<head>
<title>Typefaces on UK motorways</title>
<meta name="keywords" content="british,type face,font,fonts,highway,highways">
</head>
<body>
...Many search engines do not consider such keywords, because this feature has historically been used unreliably and even misleadingly as a way to spam search engine results in a way that is not helpful for users.
To obtain the list of keywords that the author has specified as applicable to the page, the user agent must run the following steps:
Let keywords be an empty list.
For each meta element with a name
attribute and a content attribute and where the name attribute value is an ASCII case-insensitive
match for keywords:
Split the value of the element's content attribute on commas.
Add the resulting tokens, if any, to keywords.
Remove any duplicates from keywords.
Return keywords. This is the list of keywords that the author has specified as applicable to the page.
User agents should not use this information when there is insufficient confidence in the reliability of the value.
For instance, it would be reasonable for a content management system to use the keyword information of pages within the system to populate the index of a site-specific search engine, but a large-scale content aggregator that used this information would likely find that certain users would try to game its ranking mechanism through the use of inappropriate keywords.
referrerThe value must be a referrer policy, which defines the default referrer
policy for the Document. [REFERRERPOLICY]
If any meta elements are inserted into the document or removed from the document, or existing
meta elements have their name or content attributes changed, user agents must run the
following algorithm:
Let candidate elements be the list of all meta elements that
meet the following criteria, in tree order:
name attribute, whose value is
an ASCII case-insensitive match for referrercontent attribute, whose value
is not the empty stringFor each element in candidate elements:
Let value be the value of element's content attribute, converted to ASCII
lowercase.
If value is one of the values given in the first column of the following table, then set value to the value given in the second column:
| Legacy value | Referrer policy |
|---|---|
never
| no-referrer
|
default
| the default referrer policy |
always
| unsafe-url
|
origin-when-crossorigin
| origin-when-cross-origin
|
If value is a referrer policy, then set element's node document's policy container's referrer policy to policy.
The fact that these steps are applied for each element enables deployment of fallback values for older user agents. [REFERRERPOLICY]
theme-colorThe value must be a string that matches the CSS <color> production, defining a suggested color that user agents should use to customize the display of the page or of the surrounding user interface. For example, a browser might color the page's title bar with the specified value, or use it as a color highlight in a tab bar or task switcher.
Within an HTML document, the media attribute value must
be unique amongst all the meta elements with their name attribute value set to an ASCII
case-insensitive match for theme-color.
This standard itself uses "WHATWG green" as its theme color:
<!DOCTYPE HTML>
<title>HTML Standard</title>
<meta name="theme-color" content="#3c790a">
...The media attribute may be used to describe the context
in which the provided color should be used.
If we only wanted to use "WHATWG green" as this standard's theme color in dark mode,
we could use the prefers-color-scheme media feature:
<!DOCTYPE HTML>
<title>HTML Standard</title>
<meta name="theme-color" content="#3c790a" media="(prefers-color-scheme: dark)">
...To obtain a page's theme color, user agents must run the following steps:
Let candidate elements be the list of all meta elements that
meet the following criteria, in tree order:
name attribute, whose value is
an ASCII case-insensitive match for theme-colorcontent attributeFor each element in candidate elements:
If element has a media attribute
and the value of element's media
attribute does not match the environment, then
continue.
Let value be the result of stripping leading and trailing ASCII whitespace from the value of
element's content attribute.
Let color be the result of parsing value.
If color is not failure, then return color.
Return nothing (the page has no theme color).
If any meta elements are inserted into the document or removed from the document, or existing meta elements have their
name, content, or
media attributes changed, or if the environment changes
such that any meta element's media
attribute's value may now or may no longer match the
environment, user agents must re-run the above algorithm and apply the result to any
affected UI.
When using the theme color in UI, user agents may adjust it in implementation-specific ways to make it more suitable for the UI in question. For example, if a user agent intends to use the theme color as a background and display white text over it, it might use a darker variant of the theme color in that part of the UI, to ensure adequate contrast.
color-schemeTo aid user agents in rendering the page background with the desired color scheme immediately
(rather than waiting for all CSS in the page to load), a 'color-scheme' value can
be provided in a meta element.
The value must be a string that matches the syntax for the CSS 'color-scheme' property value. It determines the page's supported color-schemes.
There must not be more than one meta element with its name attribute value set to an
ASCII case-insensitive match for color-scheme per document.
The following declaration indicates that the page is aware of and can handle a color scheme with dark background colors and light foreground colors:
<meta name="color-scheme" content="dark">To obtain a page's supported color-schemes, user agents must run the following steps:
Let candidate elements be the list of all meta elements that
meet the following criteria, in tree order:
name attribute, whose value is
an ASCII case-insensitive match for
color-schemecontent attributeFor each element in candidate elements:
content attribute.Return null.
If any meta elements are inserted into the document or removed from the document, or existing meta elements have their
name or content
attributes changed, user agents must re-run the above algorithm.
Because these rules check successive elements until they find a match, an author can provide multiple such values to handle fallback for legacy user agents. Opposite to how CSS fallback works for properties, the multiple meta elements needs to be arranged with the legacy values after the newer values.
Anyone can create and use their own extensions to the predefined set of metadata names. There is no requirement to register such extensions.
However, a new metadata name should not be created in any of the following cases:
If either the name is a URL, or the value of its accompanying content attribute is a URL; in those cases,
registering it as an extension to the predefined set of
link types is encouraged (rather than creating a new metadata name).
If the name is for something expected to have processing requirements in user agents; in that case it ought to be standardized.
Also, before creating and using a new metadata name, consulting the WHATWG Wiki MetaExtensions page is encouraged — to avoid choosing a metadata name that's already in use, and to avoid duplicating the purpose of any metadata names that are already in use, and to avoid new standardized names clashing with your chosen name. [WHATWGWIKI]
Anyone is free to edit the WHATWG Wiki MetaExtensions page at any time to add a metadata name. New metadata names can be specified with the following information:
The actual name being defined. The name should not be confusingly similar to any other defined name (e.g. differing only in case).
A short non-normative description of what the metadata name's meaning is, including the format the value is required to be in.
A list of other names that have exactly the same processing requirements. Authors should not use the names defined to be synonyms (they are only intended to allow user agents to support legacy content). Anyone may remove synonyms that are not used in practice; only names that need to be processed as synonyms for compatibility with legacy content are to be registered in this way.
One of the following:
If a metadata name is found to be redundant with existing values, it should be removed and listed as a synonym for the existing value.
If a metadata name is added in the "proposed" state for a period of a month or more without being used or specified, then it may be removed from the WHATWG Wiki MetaExtensions page.
If a metadata name is added with the "proposed" status and found to be redundant with existing values, it should be removed and listed as a synonym for the existing value. If a metadata name is added with the "proposed" status and found to be harmful, then it should be changed to "discontinued" status.
Anyone can change the status at any time, but should only do so in accordance with the definitions above.
When the http-equiv attribute is specified on a
meta element, the element is a pragma directive.
The http-equiv attribute is an enumerated
attribute. The following table lists the keywords defined for this attribute. The states
given in the first cell of the rows with keywords give the states to which those keywords map.
Some of the keywords are non-conforming, as noted in the last
column.
| State | Keyword | Notes |
|---|---|---|
| Content Language | content-language
| Non-conforming |
| Encoding declaration | content-type
| |
| Default style | default-style
| |
| Refresh | refresh
| |
| Set-Cookie | set-cookie
| Non-conforming |
| X-UA-Compatible | x-ua-compatible
| |
| Content security policy | content-security-policy
|
When a meta element is inserted
into the document, if its http-equiv attribute is
present and represents one of the above states, then the user agent must run the algorithm
appropriate for that state, as described in the following list:
http-equiv="content-language")
This feature is non-conforming. Authors are encouraged to use the lang attribute instead.
This pragma sets the pragma-set default language. Until such a pragma is successfully processed, there is no pragma-set default language.
If the element's content attribute contains a
U+002C COMMA character (,) then return.
Let input be the value of the element's content attribute.
Let position point at the first character of input.
Skip ASCII whitespace within input given position.
Collect a sequence of code points that are not ASCII whitespace from input given position.
Let candidate be the string that resulted from the previous step.
If candidate is the empty string, return.
Set the pragma-set default language to candidate.
If the value consists of multiple space-separated tokens, tokens after the first are ignored.
This pragma is almost, but not quite, entirely unlike the HTTP `Content-Language` header of the same name. [HTTP]
http-equiv="content-type")
The Encoding declaration state is
just an alternative form of setting the charset
attribute: it is a character encoding declaration. This state's user
agent requirements are all handled by the parsing section of the specification.
For meta elements with an http-equiv
attribute in the Encoding declaration
state, the content attribute must have a value
that is an ASCII case-insensitive match for a string that consists of: the literal
string "text/html;", optionally followed by any number of ASCII
whitespace, followed by the literal string "charset=utf-8".
A document must not contain both a meta element with an http-equiv attribute in the Encoding declaration state and a
meta element with the charset attribute
present.
The Encoding declaration state may be
used in HTML documents, but elements with an http-equiv attribute in that state must not be used in
XML documents.
http-equiv="default-style")
This pragma sets the name of the default CSS style sheet set.
If the meta element has no content
attribute, or if that attribute's value is the empty string, then return.
Change the preferred CSS style sheet set name with the name being the value
of the element's content attribute. [CSSOM]
http-equiv="refresh")
This pragma acts as timed redirect.
A Document object has an associated will declaratively
refresh (a boolean). It is initially false.
If the meta element has no content
attribute, or if that attribute's value is the empty string, then return.
Let input be the value of the element's content attribute.
Run the shared declarative refresh steps with the meta
element's node document, input, and the meta
element.
The shared declarative refresh steps, given a Document object
document, string input, and optionally a meta element
meta, are as follows:
If document's will declaratively refresh is true, then return.
Let position point at the first code point of input.
Skip ASCII whitespace within input given position.
Let time be 0.
Collect a sequence of code points that are ASCII digits from input given position, and let the result be timeString.
If timeString is the empty string, then:
If the code point in input pointed to by position is not U+002E (.), then return.
Otherwise, set time to the result of parsing timeString using the rules for parsing non-negative integers.
Collect a sequence of code points that are ASCII digits and U+002E FULL STOP characters (.) from input given position. Ignore any collected characters.
Let urlRecord be document's URL.
If position is not past the end of input, then:
If the code point in input pointed to by position is not U+003B (;), U+002C (,), or ASCII whitespace, then return.
Skip ASCII whitespace within input given position.
If the code point in input pointed to by position is U+003B (;) or U+002C (,), then advance position to the next code point.
Skip ASCII whitespace within input given position.
If position is not past the end of input, then:
Let urlString be the substring of input from the code point at position to the end of the string.
If the code point in input pointed to by position is U+0055 (U) or U+0075 (u), then advance position to the next code point. Otherwise, jump to the step labeled skip quotes.
If the code point in input pointed to by position is U+0052 (R) or U+0072 (r), then advance position to the next code point. Otherwise, jump to the step labeled parse.
If the code point in input pointed to by position is U+004C (L) or U+006C (l), then advance position to the next code point. Otherwise, jump to the step labeled parse.
Skip ASCII whitespace within input given position.
If the code point in input pointed to by position is U+003D (=), then advance position to the next code point. Otherwise, jump to the step labeled parse.
Skip ASCII whitespace within input given position.
Skip quotes: If the code point in input pointed to by position is U+0027 (') or U+0022 ("), then let quote be that code point, and advance position to the next code point. Otherwise, let quote be the empty string.
Set urlString to the substring of input from the code point at position to the end of the string.
If quote is not the empty string, and there is a code point in urlString equal to quote, then truncate urlString at that code point, so that it and all subsequent code points are removed.
Parse: Parse urlString relative to document. If that fails, return. Otherwise, set urlRecord to the resulting URL record.
Set document's will declaratively refresh to true.
Perform one or more of the following steps:
After the refresh has come due (as defined below), if the user has not canceled the
redirect and, if meta is given, document's active sandboxing
flag set does not have the sandboxed automatic features browsing context
flag set, then navigate
document's browsing context to
urlRecord, with historyHandling set to "replace" and the source browsing context set to
document's browsing context.
For the purposes of the previous paragraph, a refresh is said to have come due as soon as the later of the following two conditions occurs:
It is important to use document here, and not meta's
node document, as that might have changed between the initial set of steps and
the refresh coming due and meta is not always given (in case of the HTTP
`Refresh` header).
Provide the user with an interface that, when selected, navigates a browsing context to urlRecord, with document's browsing context as the source browsing context.
Do nothing.
In addition, the user agent may, as with anything, inform the user of any and all aspects of its operation, including the state of any timers, the destinations of any timed redirects, and so forth.
For meta elements with an http-equiv
attribute in the Refresh state, the content attribute must have a value consisting either of:
URL",
followed by a U+003D EQUALS SIGN character (=), followed by a valid URL string
that does not start with a literal U+0027 APOSTROPHE (') or U+0022 QUOTATION MARK (")
character.In the former case, the integer represents a number of seconds before the page is to be reloaded; in the latter case the integer represents a number of seconds before the page is to be replaced by the page at the given URL.
A news organization's front page could include the following markup in the page's
head element, to ensure that the page automatically reloads from the server every
five minutes:
<meta http-equiv="Refresh" content="300">A sequence of pages could be used as an automated slide show by making each page refresh to the next page in the sequence, using markup such as the following:
<meta http-equiv="Refresh" content="20; URL=page4.html">http-equiv="set-cookie")
This pragma is non-conforming and has no effect.
User agents are required to ignore this pragma.
http-equiv="x-ua-compatible")
In practice, this pragma encourages Internet Explorer to more closely follow the specifications.
For meta elements with an http-equiv
attribute in the X-UA-Compatible state, the
content attribute must have a value that is an
ASCII case-insensitive match for the string "IE=edge".
User agents are required to ignore this pragma.
http-equiv="content-security-policy")
This pragma enforces a Content Security
Policy on a Document. [CSP]
If the meta element is not a child of a head element,
return.
If the meta element has no content
attribute, or if that attribute's value is the empty string, then return.
Let policy be the result of executing Content Security Policy's parse
a serialized Content Security Policy algorithm on the meta element's
content attribute's value, with a source of "meta",
and a disposition of "enforce".
Remove all occurrences of the report-uri, frame-ancestors, and sandbox directives from policy.
Enforce the policy policy.
For meta elements with an http-equiv
attribute in the Content security
policy state, the content attribute must have a
value consisting of a valid Content Security
Policy, but must not contain any report-uri,
frame-ancestors, or sandbox directives.
The Content Security Policy given in the content attribute will be enforced upon the current document. [CSP]
A page might choose to mitigate the risk of cross-site scripting attacks by preventing the execution of inline JavaScript, as well as blocking all plugin content, using a policy such as the following:
<meta http-equiv="Content-Security-Policy" content="script-src 'self'; object-src 'none'">There must not be more than one meta element with any particular state in the
document at a time.
A character encoding declaration is a mechanism by which the character encoding used to store or transmit a document is specified.
The Encoding standard requires use of the UTF-8 character
encoding and requires use of the "utf-8" encoding label
to identify it. Those requirements necessitate that the document's character encoding
declaration, if it exists, specifies an encoding label using an ASCII
case-insensitive match for "utf-8". Regardless of whether a
character encoding declaration is present or not, the actual character encoding used to encode the document must be
UTF-8. [ENCODING]
To enforce the above rules, authoring tools must default to using UTF-8 for newly-created documents.
The following restrictions also apply:
In addition, due to a number of restrictions on meta elements, there can only be
one meta-based character encoding declaration per document.
If an HTML document does not start with a BOM, and its
encoding is not explicitly given by Content-Type
metadata, and the document is not an iframe srcdoc document, then the encoding must be specified
using a meta element with a charset attribute
or a meta element with an http-equiv
attribute in the Encoding declaration
state.
A character encoding declaration is required (either in the Content-Type metadata or explicitly in the file) even when all characters are in the ASCII range, because a character encoding is needed to process non-ASCII characters entered by the user in forms, in URLs generated by scripts, and so forth.
Using non-UTF-8 encodings can have unexpected results on form submission and URL encodings, which use the document's character encoding by default.
If the document is an iframe srcdoc
document, the document must not have a character encoding declaration. (In
this case, the source is already decoded, since it is part of the document that contained the
iframe.)
In XML, the XML declaration should be used for inline character encoding information, if necessary.
In HTML, to declare that the character encoding is UTF-8, the author could
include the following markup near the top of the document (in the head element):
<meta charset="utf-8">In XML, the XML declaration would be used instead, at the very top of the markup:
<?xml version="1.0" encoding="utf-8"?>style elementSupport in all current engines.
Support in all current engines.
noscript element that is a child of a head element.mediatitle attribute has special semantics on this element.[Exposed=Window]
interface HTMLStyleElement : HTMLElement {
[HTMLConstructor] constructor();
[CEReactions] attribute DOMString media;
// also has obsolete members
};
HTMLStyleElement includes LinkStyle;The style element allows authors to embed CSS style sheets in their documents.
The style element is one of several inputs to the styling processing
model. The element does not represent content for the
user.
The media attribute
says which media the styles apply to. The value must be a valid media query list.
The user agent must apply the styles when the media attribute's value matches the environment and
the other relevant conditions apply, and must not apply them otherwise.
The styles might be further limited in scope, e.g. in CSS with the use of @media blocks. This specification does not override such further restrictions or
requirements.
The default, if the media
attribute is omitted, is "all", meaning that by default styles apply to all
media.
The title attribute on style elements defines
CSS style sheet sets. If the style element
has no title attribute, then it has no title; the title attribute of ancestors does not apply to the style
element. If the style element is not in a document tree, then the title attribute is ignored. [CSSOM]
The title attribute on style
elements, like the title attribute on link
elements, differs from the global title attribute in that a
style block without a title does not inherit the title of the parent element: it
merely has no title.
The child text content of a style element must be that of a
conformant style sheet.
The user agent must run the update a style block algorithm whenever
one of the following conditions occur:
The element is popped off the stack of open elements of an HTML parser or XML parser.
The element is not on the stack of open elements of an HTML parser or XML parser, and it becomes connected or disconnected.
The element's children changed steps run.
The update a style block algorithm is as follows:
Let element be the style element.
If element has an associated CSS style sheet, remove the CSS style sheet in question.
If element is not connected, then return.
If element's type attribute is present and
its value is neither the empty string nor an ASCII case-insensitive match for
"text/css", then return.
In particular, a type value with
parameters, such as "text/css; charset=utf-8", will cause this algorithm
to return early.
If the Should element's inline behavior be blocked by Content Security
Policy? algorithm returns "Blocked" when executed upon the
style element, "style", and the style
element's child text content, then return. [CSP]
Create a CSS style sheet with the following properties:
element
The media attribute of element.
This is a reference to the (possibly absent at this time) attribute, rather than a copy of the attribute's current value. CSSOM defines what happens when the attribute is dynamically set, changed, or removed.
The title attribute of element, if
element is in a document tree, or the empty string otherwise.
Again, this is a reference to the attribute.
Unset.
Set.
null
Left at its default value.
Left uninitialized.
This doesn't seem right. Presumably we should be using the element's child text content? Tracked as issue #2997.
Once the attempts to obtain the style sheet's critical subresources, if any, are complete, or, if the style sheet has no critical subresources, once the style sheet has been parsed and processed, the user agent must run these steps:
Let element be the style element associated with the style sheet
in question.
Let success be true.
If the attempts to obtain any of the style sheet's critical subresources failed for any reason (e.g., DNS error, HTTP 404 response, a connection being prematurely closed, unsupported Content-Type), set success to false.
Note that content-specific errors, e.g., CSS parse errors or PNG decoding errors, do not affect success.
Queue an element task on the networking task source given element and the following steps:
If success is true, fire an event
named load at element.
Otherwise, fire an event named error at element.
If element contributes a script-blocking style sheet:
Assert: element's node document's script-blocking style sheet counter is greater than 0.
Decrement element's node document's script-blocking style sheet counter by 1.
The element must delay the load event of the element's node document until all the attempts to obtain the style sheet's critical subresources, if any, are complete.
This specification does not specify a style system, but CSS is expected to be supported by most web browsers. [CSS]
Support in all current engines.
The media IDL
attribute must reflect the content attribute of the same name.
The LinkStyle interface is also implemented by this element. [CSSOM]
The following document has its stress emphasis styled as bright red text rather than italics text, while leaving titles of works and Latin words in their default italics. It shows how using appropriate elements enables easier restyling of documents.
<!DOCTYPE html>
<html lang="en-US">
<head>
<title>My favorite book</title>
<style>
body { color: black; background: white; }
em { font-style: normal; color: red; }
</style>
</head>
<body>
<p>My <em>favorite</em> book of all time has <em>got</em> to be
<cite>A Cat's Life</cite>. It is a book by P. Rahmel that talks
about the <i lang="la">Felis catus</i> in modern human society.</p>
</body>
</html>If the style sheet referenced no other resources (e.g., it was an internal style sheet given by
a style element with no @import rules), then the style rules
must be immediately made available to script; otherwise, the style rules must only be
made available to script once the event loop reaches its update the
rendering step.
An element el in the context of a
Document of an HTML parser or XML parser contributes a
script-blocking style sheet if all of the following conditions are true:
el was created by that Document's parser.
el is either a style element or a link element that
was an external resource link that contributes to the styling
processing model when the el was created by the parser.
If the el is a link element, it's media attribute's value matches the
environment.
el's style sheet was enabled when the element was created by the parser.
The last time the event loop reached step 1,
el's root was that Document.
The user agent hasn't given up on loading that particular style sheet yet. A user agent may give up on loading a style sheet at any time.
Giving up on a style sheet before the style sheet loads, if the style sheet eventually does still load, means that the script might end up operating with incorrect information. For example, if a style sheet sets the color of an element to green, but a script that inspects the resulting style is executed before the sheet is loaded, the script will find that the element is black (or whatever the default color is), and might thus make poor choices (e.g., deciding to use black as the color elsewhere on the page, instead of green). Implementers have to balance the likelihood of a script using incorrect information with the performance impact of doing nothing while waiting for a slow network request to finish.
It is expected that counterparts to the above rules also apply to
<?xml-stylesheet?> PIs and HTTP `Link` headers.
However, this has not yet been thoroughly investigated.
A Document has a script-blocking style sheet counter, which is a
number, initially 0.
A Document has a style sheet that is blocking scripts if its
script-blocking style sheet counter is greater than 0, or if that
Document has a non-null browsing context
whose container document is non-null and has a
script-blocking style sheet counter greater than 0.
A Document has no style sheet that is blocking scripts if it does not
have a style sheet that is blocking
scripts as defined in the previous paragraph.