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. [Section 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. [Section 16.3.4]
Well-formed XHTML documents, like HTML standard ones, must also conform to proper nesting. No argument there. [Section 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 Chapter 15 and Chapter 16, in which we detail the XML and XHTML standards (and the Forces of Conformity).