HTML, which stands for Hyper Text Markup Language, is a descriptive language. XHTML, which stands for Extensible HyperText Markup Language, is also a descriptive language, similar to HTML in terms of presentation, but with stricter syntax. In terms of relationship, HTML is an application based on the Standard Generalized Markup Language (SGML), XML is a subset of SGML, and XHTML is based on XML.
XHTMLis based on the Extensible Markup Language (XML).HTMLis based on the Standard Generalized Markup Language (SGML).
HTMLhas looser syntax requirements.XHTMLrequires elements to be closed, including empty tags.XHTMLtag names must be in lowercase.XHTMLdocuments must have a root element.XHTMLtag order must be correct.XHTMLdocuments require all attributes to have a value, and attribute values cannot be abbreviated.XHTMLrequires all attributes to be enclosed in double quotation marks ("").XHTMLdocuments need to encode special characters such as<,>,&, etc.XHTMLdocuments cannot use--in comment content.XHTMLimages must have alt text.XHTMLdocuments use theidattribute instead of thenameattribute.
XHTMLcan incorporate variousXMLapplications, such asMathMLandSVG.HTML4.0cannot incorporate otherXMLapplications, but inHTML5, it is possible to embed applications such asMathMLandSVG.
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