XHTML 1.0: Where XML and HTML meet (2/8) - exploring XML

Which old HTML sins to overcome?

The Ten Commandments of XHTML are:

Fix up all documents that are not well-formed
Well-formedness is a new concept introduced by XML. Essentially this means that all elements must be properly nested. Although overlapping is illegal in SGML, it was widely tolerated in existing browsers.

NOT: <p>this is <em>emphasized</p></em>
BUT: <p>this is <em>emphasized</em></p>

Change all tags and attribute names to lower case
XHTML documents must use lower case for all HTML element and attribute names. XML is case-sensitive, so for instance <li> and <LI> are different tags.

Add end tags for non-empty elements
In SGML-based HTML 4 certain elements were permitted to omit the end tag; with the elements that followed implying closure. This omission is not permitted in XML-based XHTML. All elements other than those declared in the DTD as EMPTY must have an end tag.

NOT: <p>one<p>two
BUT: <p>one</p><p>two</p>

Quote all attribute values
All attribute values must be quoted, even those which appear to be numeric.

NOT: <table rows=3>
BUT: <table rows="3">

Unminimize attributes
XML does not support attribute minimization, attribute-value pairs must be written in full. Attribute names such as compact and checked cannot occur in elements without their value being specified.

NOT: <dl compact>
BUT: <dl compact="compact">

Correctly tag empty elements
Empty elements must either have an end tag or the start tag must end with />. For instance, <br/> or <hr></hr>. See below for information on ways to ensure this is backward compatible with HTML 4 user agents.