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4.14 Matching HTML elements using selectors

4.14.1 Case-sensitivity

The Selectors specification leaves the case-sensitivity of IDs, classes, element names, attribute names, and attribute values to be defined by the host language. [SELECTORS]

The unique identifier of HTML elements in documents that are in quirks mode must be treated as ASCII case-insensitive for the purposes of selector matching.

Classes from the class attribute of HTML elements in documents that are in quirks mode must be treated as ASCII case-insensitive for the purposes of selector matching.

When comparing a CSS element type selector to the names of HTML elements in HTML documents, the CSS element type selector must first be converted to ASCII lowercase. The same selector when compared to other elements must be compared according to its original case. In both cases, the comparison is case-sensitive.

When comparing the name part of a CSS attribute selector to the names of namespace-less attributes on HTML elements in HTML documents, the name part of the CSS attribute selector must first be converted to ASCII lowercase. The same selector when compared to other attributes must be compared according to its original case. In both cases, the comparison is case-sensitive.

Everything else (attribute values on HTML elements, IDs and classes in no-quirks mode and limited-quirks mode, and element names, attribute names, and attribute values in XML documents) must be treated as case-sensitive for the purposes of selector matching.

4.14.2 Pseudo-classes

There are a number of dynamic selectors that can be used with HTML. This section defines when these selectors match HTML elements. [SELECTORS]

:link

:visited

All a elements that have an href attribute, all area elements that have an href attribute, and all link elements that have an href attribute, must match one of :link and :visited.

Other specifications might apply more specific rules regarding how these elements are to match these pseudo-classes, to mitigate some privacy concerns that apply with straightforward implementations of this requirement.

:active

The :active pseudo-class is defined to match an element while an element is being activated by the user. For the purposes of defining the :active pseudo-class only, an HTML user agent must consider an element as being activated if it is:

• An element falling into one of the following categories between the time the user begins to indicate an intent to trigger the element's activation behavior and either the time the user stops indicating an intent to trigger the element's activation behavior, or the time the element's activation behavior has finished running, which ever comes first:

  • a elements that have an href attribute
  • area elements that have an href attribute
  • link elements that have an href attribute
  • button elements that are not disabled
  • input elements whose type attribute is in the Submit Button, Image Button, Reset Button, or Button state
  • elements that have their tabindex focus flag set

  For example, if the user is using a keyboard to push a button element by pressing the space bar, the element would match this pseudo-class in between the time that the element received the keydown event and the time the element received the keyup event.

• An element that the user indicates using a pointing device while that pointing device is in the "down" state (e.g. for a mouse, between the time the mouse button is pressed and the time it is depressed).

• An element that has a descendant that is currently matching the :active pseudo-class.

:hover

The :hover pseudo-class is defined to match an element while the user designates an element with a pointing device. For the purposes of defining the :hover pseudo-class only, an HTML user agent must consider an element as being one that the user designates if it is:

• An element that the user indicates using a pointing device.

• An element that has a descendant that the user indicates using a pointing device.

• An element that is the labeled control of a label element that is currently matching :hover.

Consider in particular a fragment such as:

<p> <label for=c> <input id=a> </label> <span id=b> <input id=c> </span> </p>

If the user designates the element with ID "a" with their pointing device, then the p element (and all its ancestors not shown in the snippet above), the label element, the element with ID "a", and the element with ID "c" will match the :hover pseudo-class. The element with ID "a" matches it from condition 1, the label and p elements match it because of condition 2 (one of their descendants is designated), and the element with ID "c" matches it through condition 3 (its label element matches :hover). However, the element with ID "b" does not match :hover: its descendant is not designated, even though it matches :hover.

:enabled

The :enabled pseudo-class must match any element falling into one of the following categories:

• a elements that have an href attribute
• area elements that have an href attribute
• link elements that have an href attribute
• button elements that are not disabled
• input elements that are not disabled
• select elements that are not disabled
• textarea elements that are not disabled
• optgroup elements that do not have a disabled attribute
• option elements that are not disabled
• fieldset elements that do not have a disabled attribute

:disabled

The :disabled pseudo-class must match any element that is actually disabled.

:checked

The :checked pseudo-class must match any element falling into one of the following categories:

• input elements whose type attribute is in the Checkbox state and whose checkedness state is true
• input elements whose type attribute is in the Radio Button state and whose checkedness state is true
• option elements whose selectedness is true

:indeterminate

The :indeterminate pseudo-class must match any element falling into one of the following categories:

• input elements whose type attribute is in the Checkbox state and whose indeterminate IDL attribute is set to true
• input elements whose type attribute is in the Radio Button state and whose radio button group contains no input elements whose checkedness state is true.
• progress elements with no value content attribute

:default

The :default pseudo-class must match any element falling into one of the following categories:

• button elements that are their form's default button
• input elements whose type attribute is in the Submit Button or Image Button state, and that are their form's default button
• input elements to which the checked attribute applies and that have a checked attribute
• option elements that have a selected attribute

:valid

The :valid pseudo-class must match any element falling into one of the following categories:

• elements that are candidates for constraint validation and that satisfy their constraints
• form elements that are not the form owner of any elements that themselves are candidates for constraint validation but do not satisfy their constraints
• fieldset elements that have no descendant elements that themselves are candidates for constraint validation but do not satisfy their constraints

:invalid

The :invalid pseudo-class must match any element falling into one of the following categories:

• elements that are candidates for constraint validation but that do not satisfy their constraints
• form elements that are the form owner of one or more elements that themselves are candidates for constraint validation but do not satisfy their constraints
• fieldset elements that have of one or more descendant elements that themselves are candidates for constraint validation but do not satisfy their constraints

:in-range

The :in-range pseudo-class must match all elements that are candidates for constraint validation, have range limitations, and that are neither suffering from an underflow nor suffering from an overflow.

:out-of-range

The :out-of-range pseudo-class must match all elements that are candidates for constraint validation, have range limitations, and that are either suffering from an underflow or suffering from an overflow.

:required

The :required pseudo-class must match any element falling into one of the following categories:

• input elements that are required
• select elements that have a required attribute
• textarea elements that have a required attribute

:optional

The :optional pseudo-class must match any element falling into one of the following categories:

• input elements to which the required attribute applies that are not required
• select elements that do not have a required attribute
• textarea elements that do not have a required attribute

:read-only

:read-write

The :read-write pseudo-class must match any element falling into one of the following categories, which for the purposes of Selectors are thus considered user-alterable: [SELECTORS]

• input elements to which the readonly attribute applies, and that are mutable (i.e. that do not have the readonly attribute specified and that are not disabled)
• textarea elements that do not have a readonly attribute, and that are not disabled
• elements that are editing hosts or editable and are neither input elements nor textarea elements

The :read-only pseudo-class must match all other HTML elements.

Another section of this specification defines the target element used with the :target pseudo-class.

This specification does not define when an element matches the :focus or :lang() dynamic pseudo-classes, as those are all defined in sufficient detail in a language-agnostic fashion in the Selectors specification. [SELECTORS]