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In HTML syntax, an attribute is added to a 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 ...
HTML attributes define desired behavior or indicate additional element properties. Most attributes require a value. In HTML, the value can be left unquoted if it does not include spaces (attribute=value), or it can be quoted with single or double quotes (attribute='value' or attribute="value"). In XML, those quotes are required.
HTML markup consists of several key components, including those called tags (and their attributes), character-based data types, character references and entity references. HTML tags most commonly come in pairs like < h1 > and </ h1 >, although some represent empty elements and so are unpaired, for example < img >.
The meta element has two uses: either to emulate the use of an HTTP response header field, or to embed additional metadata within the HTML document. With HTML up to and including HTML 4.01 and XHTML, there were four valid attributes: content, http-equiv, name and scheme. Under HTML 5, charset has been added and scheme has been removed.
In SGML, HTML and XML documents, the logical constructs known as character data and attribute values consist of sequences of characters, in which each character can manifest directly (representing itself), or can be represented by a series of characters called a character reference, of which there are two types: a numeric character reference and a character entity reference.
For these reasons, and in support of a more semantic web, attributes attached to elements within HTML should describe their semantic purpose, rather than merely their intended display properties in one particular medium.
The alt attribute is the HTML attribute used in HTML and XHTML documents to specify alternative text (alt text) that is to be displayed in place of an element that cannot be rendered. The alt attribute is used for short descriptions, with longer descriptions using the longdesc attribute .
Examples of this include nesting a "ul" element directly inside another "ul" element for any of the HTML 4.01 or XHTML DTDs. Dan Connolly cites the use of title element outside the head section. [1] Use of proprietary or undefined elements and attributes instead of those defined in W3C recommendations.