HTML and CSS Reference
In-Depth Information
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
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,
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 differentlyper-
haps 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 Forces of Conformity).