HTML attributes are special words used to adjust the behavior or display of an HTML element. 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 many element types; and (4) event attributes used to cause element types to specify scripts to be run under specific circumstances.

Doctype HTML is a declaration that tells the browser what version of HTML the document is written in.

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 element 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).

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