HTML and CSS Reference
3.4. 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
tageven 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, ac-
cording 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 con-
form to proper nesting. No argument there. [ Correctly Nested Elements,
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 differentlyper-
haps in tabular form, with red, white, and blue stripes and accompanying
XML and XHTML standards (and the Forces of Conformity).