HTML attribute

An HTML attribute is a modifier of an HTML element type. An attribute either modifies the default functionality of an element type or provides functionality to certain element types unable to function correctly without them. In HTML syntax, an attribute is added to an HTML start tag.

Several basic attributes types have been recognized, including: (1) required attributes, needed by a particular element type for that element type to function correctly; (2) optional attributes, used to modify the default functionality of an element type; (3) standard attributes, supported by a large number of element types; and (4) event attributes, used to cause element types to specify scripts to be run under specific circumstances.

Some attribute types function differently when used to modify different element types. For example, the attribute name is used by several element types, but has slightly different functions in each.

HTML attributes generally appear as name-value pairs, separated by =, and are written within the start tag of an element, after the element's name:

Where tag names the HTML element type, and attribute is the name of the attribute, set to the provided value. The value may be enclosed in single or double quotes, although values consisting of certain characters can be left unquoted in HTML (but not XHTML). Leaving attribute values unquoted is considered unsafe.

Although most attributes are provided as paired names and values, some affect the element simply by their presence in the start tag of the element (like the ismap attribute for the img element).

Most elements can take any of several common attributes:

The abbreviation element, abbr, can be used to demonstrate these various attributes:

This example displays as HTML; in most browsers, pointing the cursor at the abbreviation should display the title text "Hypertext Markup Language."

Most elements also take the language-related attributes lang and dir.

HTML attributes are generally classed as required attributes, optional attributes, standard attributes, and event attributes. Usually the required and optional attributes modify specific HTML elements, while the standard attributes can be applied to most HTML elements. Event attributes, added in HTML version 4, allow an element to specify scripts to be run under specific circumstances.

Standard attributes are also known as global attributes, and function with a large number of elements. They include the basic standard attributes: these include accesskey, class, contenteditable, contextmenu, data, dir, hidden, id, lang, style, tabindex, title. There are also some experimental ones. Both xml:lang and xml:base have been deprecated. The multiple aria-* attributes improve accessibility. The event handler attributes are listed later on.

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