Well-Formed Documents and XHTML
XHTML is HTML’s prissy cousin. What would pass most beauty contests as a very proper and complete HTML document, done according to the book and including end-paragraph tags, might well be rejected by the XML judges as a malformed file.
To conform with XML, XHTML insists that documents be “well
formed.” Among other things, that means that every tag must have an
ending tag—even the ones like <br> and <hr> for which the HTML standard forbids
the use of an end tag. With XHTML, the ending is placed inside the start
tag: <br />, for example.
[Handling Empty Elements,
16.3.3]
It also means that tag and attribute names are case-sensitive and,
according to the current XHTML standard, must be in lowercase. Hence,
only <head> is acceptable, and
it is not the same as <HEAD> or <HeAd>, as it is with the HTML standard.
[Case Sensitivity,
16.3.4]
Well-formed XHTML documents, like HTML standard ones, must also conform to proper nesting. No argument there. [Correctly Nested Elements, 16.3.1]
In their defense, the XML standard and its offspring, XHTML,
emphasize extensibility. That way, <p> can mean the beginning of a
paragraph in HTML, whereas another variant of the language may define
the contents of the <P> tag to
be election-poll results that display quite differently—perhaps in
tabular form, with red, white, and blue stripes and accompanying
patriotic music.
We will discuss this further in Chapters 15 and 16, in which we detail the XML and XHTML standards (and the ...
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