CSS Object Model (CSSOM)

1. Introduction

This document formally specifies the core features of the CSS Object Model (CSSOM). Other documents in the CSSOM family of specifications as well as other CSS related specifications define extensions to these core features.

The core features of the CSSOM are oriented towards providing basic capabilities to author-defined scripts to permit access to and manipulation of style related state information and processes.

The features defined below are fundamentally based on prior specifications of the W3C DOM Working Group, primarily [DOM]. The purposes of the present document are (1) to improve on that prior work by providing more technical specificity (so as to improve testability and interoperability), (2) to deprecate or remove certain less-widely implemented features no longer considered to be essential in this context, and (3) to newly specify certain extensions that have been or expected to be widely implemented.

2. Terminology

This specification employs certain terminology from the following documents: DOM, HTML, CSS Syntax, Encoding, URL, Fetch, Associating Style Sheets with XML documents and XML. [DOM] [HTML] [CSS3SYN] [ENCODING] [URL] [FETCH] [XML-STYLESHEET] [XML]

When this specification talks about object A where A is actually an interface, it generally means an object implementing interface A.

The terms set and unset to refer to the true and false values of binary flags or variables, respectively. These terms are also used as verbs in which case they refer to mutating some value to make it true or false, respectively.

The term supported styling language refers to CSS.

Note: If another styling language becomes supported in user agents, this specification is expected to be updated as necessary.

The term supported CSS property refers to a CSS property that the user agent implements, including any vendor-prefixed properties, but excluding custom properties. A supported CSS property must be in its lowercase form for the purpose of comparisons in this specification.

In this specification the ::before and ::after pseudo-elements are assumed to exist for all elements even if no box is generated for them.

When a method or an attribute is said to call another method or attribute, the user agent must invoke its internal API for that attribute or method so that e.g. the author can’t change the behavior by overriding attributes or methods with custom properties or functions in ECMAScript.

Unless otherwise stated, string comparisons are done in a case-sensitive manner.

2.1. Common Serializing Idioms

To escape a character means to create a string of "\" (U+005C), followed by the character.

To escape a character as code point means to create a string of "\" (U+005C), followed by the Unicode code point as the smallest possible number of hexadecimal digits in the range 0-9 a-f (U+0030 to U+0039 and U+0061 to U+0066) to represent the code point in base 16, followed by a single SPACE (U+0020).

To serialize an identifier means to create a string represented by the concatenation of, for each character of the identifier:

• If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD).
• If the character is in the range [\1-\1f] (U+0001 to U+001F) or is U+007F, then the character escaped as code point.
• If the character is the first character and is in the range [0-9] (U+0030 to U+0039), then the character escaped as code point.
• If the character is the second character and is in the range [0-9] (U+0030 to U+0039) and the first character is a "-" (U+002D), then the character escaped as code point.
• If the character is the first character and is a "-" (U+002D), and there is no second character, then the escaped character.
• If the character is not handled by one of the above rules and is greater than or equal to U+0080, is "-" (U+002D) or "_" (U+005F), or is in one of the ranges [0-9] (U+0030 to U+0039), [A-Z] (U+0041 to U+005A), or [a-z] (U+0061 to U+007A), then the character itself.
• Otherwise, the escaped character.

To serialize a string means to create a string represented by '"' (U+0022), followed by the result of applying the rules below to each character of the given string, followed by '"' (U+0022):

• If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD).
• If the character is in the range [\1-\1f] (U+0001 to U+001F) or is U+007F, the character escaped as code point.
• If the character is '"' (U+0022) or "\" (U+005C), the escaped character.
• Otherwise, the character itself.

Note: "'" (U+0027) is not escaped because strings are always serialized with '"' (U+0022).

To serialize a URL means to create a string represented by "url(", followed by the serialization of the URL as a string, followed by ")".

To serialize a LOCAL means to create a string represented by "local(", followed by the serialization of the LOCAL as a string, followed by ")".

To serialize a comma-separated list concatenate all items of the list in list order while separating them by ", ", i.e., COMMA (U+002C) followed by a single SPACE (U+0020).

To serialize a whitespace-separated list concatenate all items of the list in list order while separating them by " ", i.e., a single SPACE (U+0020).

Note: When serializing a list according to the above rules, extraneous whitespace is not inserted prior to the first item or subsequent to the last item. Unless otherwise specified, an empty list is serialized as the empty string.

3. CSSOMString

Most strings in CSSOM interfaces use the CSSOMString type. Each implementation chooses to define it as either USVString or DOMString:

typedef USVString CSSOMString;

Or, alternatively:

typedef DOMString CSSOMString;

4. Media Queries

Media queries are defined by [MEDIAQUERIES]. This section defines various concepts around media queries, including their API and serialization form.

4.1. Parsing Media Queries

To parse a media query list for a given string s into a media query list is defined in the Media Queries specification. Return the list of media queries that the algorithm defined there gives.

Note: A media query that ends up being "ignored" will turn into "not all".

To parse a media query for a given string s means to follow the parse a media query list steps and return null if more than one media query is returned or a media query if a single media query is returned.

Note: Again, a media query that ends up being "ignored" will turn into "not all".

4.2. Serializing Media Queries

To serialize a media query list run these steps:

1. If the media query list is empty, then return the empty string.
2. Serialize each media query in the list of media queries, in the same order as they appear in the media query list, and then serialize the list.

To serialize a media query let s be the empty string, run the steps below:

1. If the media query is negated append "not", followed by a single SPACE (U+0020), to s.
2. Let type be the serialization as an identifier of the media type of the media query, converted to ASCII lowercase.
3. If the media query does not contain media features append type, to s, then return s.
4. If type is not "all" or if the media query is negated append type, followed by a single SPACE (U+0020), followed by "and", followed by a single SPACE (U+0020), to s.
5. Then, for each media feature:
   1. Append a "(" (U+0028), followed by the media feature name, converted to ASCII lowercase, to s.
   2. If a value is given append a ":" (U+003A), followed by a single SPACE (U+0020), followed by the serialized media feature value, to s.
   3. Append a ")" (U+0029) to s.
   4. If this is not the last media feature append a single SPACE (U+0020), followed by "and", followed by a single SPACE (U+0020), to s.
6. Return s.

not screen and (min-WIDTH:5px) AND (max-width:40px)

not screen and (min-width: 5px) and (max-width: 40px)

all and (color) and (color)

(color) and (color)

4.2.1. Serializing Media Feature Values

This should probably be done in terms of mapping it to serializing CSS values as media features are defined in terms of CSS values after all.

To serialize a media feature value named v locate v in the first column of the table below and use the serialization format described in the second column:

Other specifications can extend this table and vendor-prefixed media features can have custom serialization formats as well.

4.3. Comparing Media Queries

To compare media queries m1 and m2 means to serialize them both and return true if they are a case-sensitive match and false if they are not.

4.4. The MediaList Interface

An object that implements the MediaList interface has an associated collection of media queries.

[Exposed=Window]
interface MediaList {
  stringifier attribute [LegacyNullToEmptyString] CSSOMString mediaText;
  readonly attribute unsigned long length;
  getter CSSOMString? item(unsigned long index);
  undefined appendMedium(CSSOMString medium);
  undefined deleteMedium(CSSOMString medium);
};

The object’s supported property indices are the numbers in the range zero to one less than the number of media queries in the collection of media queries represented by the collection. If there are no such media queries, then there are no supported property indices.

To create a MediaList object with a string text, run the following steps:

1. Create a new MediaList object.
2. Set its mediaText attribute to text.
3. Return the newly created MediaList object.

The mediaText attribute, on getting, must return a serialization of the collection of media queries. Setting the mediaText attribute must run these steps:

1. Empty the collection of media queries.
2. If the given value is the empty string, then return.
3. Append all the media queries as a result of parsing the given value to the collection of media queries.

The item(index) method must return a serialization of the media query in the collection of media queries given by index, or null, if index is greater than or equal to the number of media queries in the collection of media queries.

The length attribute must return the number of media queries in the collection of media queries.

The appendMedium(medium) method must run these steps:

1. Let m be the result of parsing the given value.
2. If m is null, then return.
3. If comparing m with any of the media queries in the collection of media queries returns true, then return.
4. Append m to the collection of media queries.

The deleteMedium(medium) method must run these steps:

1. Let m be the result of parsing the given value.
2. If m is null, then return.
3. Remove any media query from the collection of media queries for which comparing the media query with m returns true. If nothing was removed, then throw a NotFoundError exception.

5. Selectors

Selectors are defined in the Selectors specification. This section mainly defines how to serialize them.

5.1. Parsing Selectors

To parse a group of selectors means to parse the value using the selectors_group production defined in the Selectors specification and return either a group of selectors if parsing did not fail or null if parsing did fail.

5.2. Serializing Selectors

To serialize a group of selectors serialize each selector in the group of selectors and then serialize a comma-separated list of these serializations.

To serialize a selector let s be the empty string, run the steps below for each part of the chain of the selector, and finally return s:

1. If there is only one simple selector in the compound selectors which is a universal selector, append the result of serializing the universal selector to s.
2. Otherwise, for each simple selector in the compound selectors that is not a universal selector of which the namespace prefix maps to a namespace that is not the default namespace serialize the simple selector and append the result to s.
3. If this is not the last part of the chain of the selector append a single SPACE (U+0020), followed by the combinator ">", "+", "~", ">>", "||", as appropriate, followed by another single SPACE (U+0020) if the combinator was not whitespace, to s.
4. If this is the last part of the chain of the selector and there is a pseudo-element, append "::" followed by the name of the pseudo-element, to s.

To serialize a simple selector let s be the empty string, run the steps below, and finally return s:

type selector

universal selector

1. If the namespace prefix maps to a namespace that is not the default namespace and is not the null namespace (not in a namespace) append the serialization of the namespace prefix as an identifier, followed by a "|" (U+007C) to s.
2. If the namespace prefix maps to a namespace that is the null namespace (not in a namespace) append "|" (U+007C) to s.
3. If this is a type selector append the serialization of the element name as an identifier to s.
4. If this is a universal selector append "*" (U+002A) to s.

attribute selector

1. Append "[" (U+005B) to s.
2. If the namespace prefix maps to a namespace that is not the null namespace (not in a namespace) append the serialization of the namespace prefix as an identifier, followed by a "|" (U+007C) to s.
3. Append the serialization of the attribute name as an identifier to s.
4. If there is an attribute value specified, append "=", "~=", "|=", "^=", "$=", or "*=" as appropriate (depending on the type of attribute selector), followed by the serialization of the attribute value as a string, to s.
5. If the attribute selector has the case-sensitivity flag present, append " i" (U+0020 U+0069) to s.
6. Append "]" (U+005D) to s.

class selector

Append a "." (U+002E), followed by the serialization of the class name as an identifier to s.

ID selector

Append a "#" (U+0023), followed by the serialization of the ID as an identifier to s.

pseudo-class

If the pseudo-class does not accept arguments append ":" (U+003A), followed by the name of the pseudo-class, to s.

Otherwise, append ":" (U+003A), followed by the name of the pseudo-class, followed by "(" (U+0028), followed by the value of the pseudo-class argument(s) determined as per below, followed by ")" (U+0029), to s.

:lang()

The serialization of a comma-separated list of each argument’s serialization as a string, preserving relative order.

:nth-child()

:nth-last-child()

:nth-of-type()

:nth-last-of-type()

The result of serializing the value using the rules to serialize an <an+b> value.

:not()

The result of serializing the value using the rules for serializing a group of selectors.

6. CSS

6.1. CSS Style Sheets

A CSS style sheet is an abstract concept that represents a style sheet as defined by the CSS specification. In the CSSOM a CSS style sheet is represented as a CSSStyleSheet object.

CSSStyleSheet(options)

When called, execute the steps to create a constructed CSSStyleSheet given options and return the result.

To create a constructed CSSStyleSheet

given CSSStyleSheetInit options, run these steps:

1. Construct a new CSSStyleSheet object sheet.
2. Set sheet’s location to the base URL of the associated Document for the current global object.
3. Set sheet’s stylesheet base URL to the baseURL attribute value from options.
4. Set sheet’s parent CSS style sheet to null.
5. Set sheet’s owner node to null.
6. Set sheet’s owner CSS rule to null.
7. Set sheet’s title to the empty string.
8. Unset sheet’s alternate flag.
9. Set sheet’s origin-clean flag.
10. Set sheet’s constructed flag.
11. Set sheet’s Constructor document to the associated Document for the current global object.
12. If the media attribute of options is a string, create a MediaList object from the string and assign it as sheet’s media. Otherwise, serialize a media query list from the attribute and then create a MediaList object from the resulting string and set it as sheet’s media.
13. If the disabled attribute of options is true, set sheet’s disabled flag.
14. Return sheet.

A CSS style sheet has a number of associated state items:

type

The literal string "text/css".

location

Specified when created. The absolute-URL string of the first request of the CSS style sheet or null if the CSS style sheet was embedded. Does not change during the lifetime of the CSS style sheet.

parent CSS style sheet

Specified when created. The CSS style sheet that is the parent of the CSS style sheet or null if there is no associated parent.

owner node

Specified when created. The DOM node associated with the CSS style sheet or null if there is no associated DOM node.

owner CSS rule

Specified when created. The CSS rule in the parent CSS style sheet that caused the inclusion of the CSS style sheet or null if there is no associated rule.

media

Specified when created. The MediaList object associated with the CSS style sheet.

If this property is specified to a string, the media must be set to the return value of invoking create a MediaList object steps for that string.

If this property is specified to an attribute of the owner node, the media must be set to the return value of invoking create a MediaList object steps for the value of that attribute. Whenever the attribute is set, changed or removed, the media’s mediaText attribute must be set to the new value of the attribute, or to null if the attribute is absent.

Note: Changing the media’s mediaText attribute does not change the corresponding attribute on the owner node.

Note: The owner node of a CSS style sheet, if non-null, is the node whose associated CSS style sheet is the CSS style sheet in question, when the CSS style sheet is added.

title

Specified when created. The title of the CSS style sheet, which can be the empty string.

In the following, the title is non-empty for the first style sheet, but is empty for the second and third style sheets.

<style title="papaya whip">
  body { background: #ffefd5; }
</style>

<style title="">
  body { background: orange; }
</style>

<style>
  body { background: brown; }
</style>

If this property is specified to an attribute of the owner node, the title must be set to the value of that attribute. Whenever the attribute is set, changed or removed, the title must be set to the new value of the attribute, or to the empty string if the attribute is absent.

Note: HTML only specifies title to be an attribute of the owner node if the node is in in a document tree.

alternate flag

Specified when created. Either set or unset. Unset by default.

The following CSS style sheets have their alternate flag set:

<?xml-stylesheet alternate="yes" title="x" href="data:text/css,…"?>

<link rel="alternate stylesheet" title="x" href="data:text/css,…">

disabled flag

Either set or unset. Unset by default.

Note: Even when unset it does not necessarily mean that the CSS style sheet is actually used for rendering.

CSS rules

The CSS rules associated with the CSS style sheet.

origin-clean flag

Specified when created. Either set or unset. If it is set, the API allows reading and modifying of the CSS rules.

constructed flag

Specified when created. Either set or unset. Unset by default. Signifies whether this stylesheet was created by invoking the IDL-defined constructor.

disallow modification flag

Either set or unset. Unset by default. If set, modification of the stylesheet’s rules is not allowed.

constructor document

Specified when created. The Document a constructed stylesheet is associated with. Null by default. Only non-null for stylesheets that have constructed flag set.

stylesheet base URL

The base URL to use when resolving relative URLs in the stylesheet. Null by default. Only non-null for stylesheets that have constructed flag set.

6.1.1. The StyleSheet Interface

The StyleSheet interface represents an abstract, base style sheet.

[Exposed=Window]
interface StyleSheet {
  readonly attribute CSSOMString type;
  readonly attribute USVString? href;
  readonly attribute (Element or ProcessingInstruction)? ownerNode;
  readonly attribute CSSStyleSheet? parentStyleSheet;
  readonly attribute DOMString? title;
  [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
  attribute boolean disabled;
};

The type attribute must return the type.

The href attribute must return the location.

The ownerNode attribute must return the owner node.

The parentStyleSheet attribute must return the parent CSS style sheet.

The title attribute must return the title or null if title is the empty string.

The media attribute must return the media.

The disabled attribute, on getting, must return true if the disabled flag is set, or false otherwise. On setting, the disabled attribute must set the disabled flag if the new value is true, or unset the disabled flag otherwise.

6.1.2. The CSSStyleSheet Interface

The CSSStyleSheet interface represents a CSS style sheet.

[Exposed=Window]
interface CSSStyleSheet : StyleSheet {
  constructor(optional CSSStyleSheetInit options = {});

  readonly attribute CSSRule? ownerRule;
  [SameObject] readonly attribute CSSRuleList cssRules;
  unsigned long insertRule(CSSOMString rule, optional unsigned long index = 0);
  undefined deleteRule(unsigned long index);

  Promise<CSSStyleSheet> replace(USVString text);
  undefined replaceSync(USVString text);
};

dictionary CSSStyleSheetInit {
  DOMString baseURL = null;
  (MediaList or DOMString) media = "";
  boolean disabled = false;
};

The ownerRule attribute must return the owner CSS rule. If a value other than null is ever returned, then that same value must always be returned on each get access.

The cssRules attribute must follow these steps:

1. If the origin-clean flag is unset, throw a SecurityError exception.
2. Return a read-only, live CSSRuleList object representing the CSS rules.

   Note: Even though the returned CSSRuleList object is read-only (from the perspective of client-authored script), it can nevertheless change over time due to its liveness status. For example, invoking the insertRule() or deleteRule() methods can result in mutations reflected in the returned object.

The insertRule(rule, index) method must run the following steps:

1. If the origin-clean flag is unset, throw a SecurityError exception.
2. If the