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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). [2] [3] Leaving attribute values unquoted is considered unsafe. [4]
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.
[74] [75] Leaving attribute values unquoted is considered unsafe. [76] In contrast with name-value pair attributes, there are some attributes that affect the element simply by their presence in the start tag of the element, [6] like the ismap attribute for the img element. [77] There are several common attributes that may appear in many elements :
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.
Many field values may contain a quality (q) key-value pair separated by equals sign, specifying a weight to use in content negotiation. [9] For example, a browser may indicate that it accepts information in German or English, with German as preferred by setting the q value for de higher than that of en, as follows: Accept-Language: de; q=1.0 ...
HTML attributes are often needed for various reasons. An attribute takes the basic form attribute="value", where combining multiple repeats this with attribute="value" attribute2="value2". Important points to realize: All table markup, except table end (|}), can have attributes added. Table and row markup ({| and |-) don't directly hold content.
In HTML DOM (Document Object Model), every element is a node: [4] A document is a document node. All HTML elements are element nodes. All HTML attributes are attribute nodes. Text inserted into HTML elements are text nodes. Comments are comment nodes.
Incorrect HTML entity escaping may also open up security vulnerabilities for injection attacks such as cross-site scripting. If HTML attributes are left unquoted, certain characters, most importantly whitespace, such as space and tab, must be escaped using entities. Other languages related to HTML have their own methods of escaping characters.