I suppose this should be updated now that the landscape has changed quite a bit:
For an HTML5 doctype, it's no longer required. For example: we leave it out of the pages here at Stack Overflow. If you're using an HTML5 doctype then it's completely optional and defaults to
For an HTML4 doctype (to be valid), you need it. For a browser to actually function, it's not strictly needed and will behave just fine (this has been true all the way back to Netscape 2) - but it won't be valid HTML. If you have an HTML4 doctype, then keep it around and be valid - cause hey, why not?
I would use
language="" however is deprecated, I'd leave it out.
Also, any validator is going to throw an error, though it will likely work inside the browser (unless you're dealing with something very old).