Backward Compatibility

One of the best reasons for you to embrace HTML5 today is that it
works in most existing browsers. Right now, even in Internet Explorer
6, you can start using HTML5 and slowly transition your markup. It’ll
even validate with the W3C’s validation service (conditionally, of course,
because the standards are still evolving).
If you’ve worked with HTML or XML, you’ve come across the doctype
declaration before. It’s used to tell validators and editors what tags and
attributes you can use and how the document should be formed. It’s
also used by a lot of web browsers to determine how the browser will
render the page. A valid doctype often causes browsers to render pages
in “standards mode.”
Compared to the rather verbose XHTML 1.0 Transitional doctype used
by many sites:

the HTML5 doctype is ridiculously simple: