The eXtensible Hypertext Markup Language is the new version of HTML. It is completely XML compliant, but what does that really mean?

The HTML standard is actually a recommendation, and to this end you have probably witnessed the different ways in which competing browser manufacturers choose to interpret the element tags. It is not uncommon for a HTML page to be rendered differently, and web page designers have for a long time tested their creations in different browsers to identify any deviations from their original intentions.

Also, the browser manufacturers have made it 'easier' for individuals to create HTML pages by allowing us to 'forget' to close some of the element tags. This would break the 'nesting' rule, but allowing it prevents novice users from getting bogged down in debugging. The downside is that it permits the production of 'sloppy' HTML code, making it un-friendly for machine reading purposes. XML is strict, and will not allow such practices.

XHTML uses the strictness of XML to tighten up the formality of HTML document preparation. A document that conforms is regarded as being well formed.