An HTML attribute is a modifier of an HTML element. 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:
<tag attribute=”value”>(content to be modified by the tag)</tag>Where tag names the HTML element, 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 id attribute provides a document-wide unique identifier for an element. This can be used as CSS selector to provide presentational properties, by browsers to focus attention on the specific element, or by scripts to alter the contents or presentation of an element. Appended to the URL of the page, the URL directly targets the specific element within the document, typically a sub-section of the page. For example, the ID “Attributes” in http://en.wikipedia.org/wiki/HTML#Attributes
The class attribute provides a way of classifying similar elements. This can be used for semantic or presentation purposes. Semantically, for example, classes are used in microformats. Presentationally, for example, an HTML document might use the designationclass=”notation” to indicate that all elements with this class value are subordinate to the main text of the document. Such elements might be gathered together and presented as footnotes on a page instead of appearing in the place where they occur in the HTML source.
An author may use the style non-attributal codes presentational properties to a particular element. It is considered better practice to use an element’s id or class attributes to select the element with a stylesheet, though sometimes this can be too cumbersome for a simple and specific or ad hoc application of styled properties.
The title attribute is used to attach subtextual explanation to an element. In most browsers this attribute is displayed as what is often referred to as a tooltip.
The abbreviation element, abbr, can be used to demonstrate these various attributes:
<abbr id=”anId” class=”aClass” style=”color:blue;” title=”Hypertext Markup Language”>HTML</abbr>
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.