XHTML/HTML Differences

In spite of the grammatical similarity between XHTML and HTML in terms of element and attribute names, there are some significant differences between these languages.

The differences are significant because they have an impact on the wellformedness constraints of XML. In other words, a document that does not comply with the syntax rules that XHTML add, will not even be wellformed, and can therefore never be valid.

If you do not remember or do not understand what wellformedness and validity is, you are advised to read the XML Syntax Introduction.