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Saturday, December 6, 2008

Differences Between XHTML and HTML

Even though this is a CSS reference, we should spend some time talking about HTML and XHTML, because your choice of markup language will affect how CSS is applied in some instances. Moreover, in order to understand the variations in the way CSS is applied to HTML and XHTML, you need to grasp the fundamental differences between the two markup languages.

The most important difference between the two markup languages is that HyperText Markup Language, or HTML, is an application of SGML (Standard Generalized Markup Language),¹ and allows an author to omit certain tags and use attribute minimization.² The Extensible HyperText Markup Language, or XHTML, is an application of XML (Extensible Markup Language).³ It doesn’t permit the omission of any tags or the use of attribute minimization. However, it provides a shorthand notation for empty elements—for example, we could use instead of —which HTML does not. A conforming XML document must be well formed, which, among other things, means that there must be an end tag for every start tag, and that nested tags must be closed in the right order.⁴When an XML parser encounters an error relating to the document’s well-formedness, it must abort, whereas an HTML parser is expected to attempt to recover and continue.

There are three areas in which the differences between HTML and XHTML affect our use of CSS:

case sensitivity
optional tags
properties for the root element

Note, though, that these differences apply only when an XHTML document is served as an application of XML; that is, with a MIME type of application/xhtml+xml, application/xml, or text/xml. An XHTML document served with a MIME type of text/html must be parsed and interpreted as HTML, so the HTML rules apply in this case. A style sheet written for an XHTML document being served with a MIME type of text/html may not work as intended if the document is then served with a MIME type of application/xhtml+xml. For more information about MIME types, make sure to read MIME Types.

This can be especially important when you’re serving XHTML documents as text/html. Unless you’re aware of the differences, you may create style sheets that won’t work as intended if the document’s served as real XHTML.

Where the terms “XHTML” and “XHTML document” appear in the remainder of this section, they refer to XHTML markup served with an XML MIME type. XHTML markup served as text/html is an HTML document as far as browsers are concerned.

Labels: html, xhtml