The differences between HTML and XHTML have been noted with a new list of deprecated items. Differences between HTML4.01 and HTML5 less so. The following identifies one difference that shall affect numerous sites. It’s images.

HTML 4.01 and HTML5 each define <image> in a similar manner. The difference occurs when HTML5 introduces “Embedded Content” which states,

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 a case study page between (X)HTML5 Conformance Checking Service Technology Preview and W3C® Unicorn “The Web’s Universal Conformance Checker – ALPHA Test Version”. It passed all W3C® image conformance variants but failed (X)HTML5’s. Test case was setup and run through the (X)HTML5 Conformance Checking Service. The Document Type Definition (DTD) was Markup Language specific, i.e., <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" ""> and <!DOCTYPE html>.

[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]