The differences between HTML and XHTML have been noted with a new list of deprecated items (where one can note that <acronym> has been included). Differences between HTML 4.01 and HTML5 less so. The following identifies one difference that shall affect numerous sites. It’s anchors.

HTML 4.01 and HTML5 each define <a> as an inline element. The difference occurs when HTML5 introduces “Significant Text” and “Embedded Content”.

Significant text
Significant text, for the purposes of determining the presence of significant inline content, consists of any character other than those falling in the Unicode categories Zs, Zl, Zp, Cc, and Cf. [UNICODE] [Elementary note: That could be “.” or a single one pixel transparent GIF.]
Embedded content
Embedded content consists of elements that introduce content from other resources into the document, for example img. Embedded content elements can have fallback content: content that is to be used when the external resource cannot be used (e.g. because it is of an unsupported format). The element definitions state what the fallback is, if any.

This study was performed after comparison of this site’s index page between (X)HTML5 Conformance Checking Service Technology Preview and W3C® Unicorn “The Web’s Universal Conformance Checker – ALPHA Test Version”. It passed W3C® anchor conformance but failed (X)HTML5’s. Test case was setup and run through the (X)HTML5 Conformance Checking Service.

[Note: The following addresses HTML5; not XHTML5. All HTML 4.01 text/html and XHTML 1.0 text/html documents are considered by Web Applications 1.0 to be HTML5. 1]