Changes in HTML 5 - <?xml?> Declaration

What's new in HTML 5

The <?xml?> declaration was introduced in the 2000 W3C standard version of HTML. However, many documents on the web are still coded without it and therefore may fall back to the older 1997 HTML version 4 standard, so going forward you should make sure that any HTML pages start with an <?xml?> declaration.

Differences between HTML 5 and earlier versions of HTML