I think you still haven't understood this warning. There isn't anything you have
to change. The validator is simply telling you: This document is interpreted as HTML5. We, the W3C, cannot guarantee for the results to be correct. It doesn't say that your document is wrong or anything.
question is: What do you want? HTML5? HTML4? XHTML? Judging from your tag style, you seem to want XHTML. But that's just a guess, it might also be that you want HTML5 with XHTML style tags. Only you know that.
If this is supposed to be HTML5, everything is OK. Just ignore the warning (but be aware that HTML5 is a work in progress with different support in different browsers). If the document is supposed to be XHTML, you need to look up the right doctype
for XHTML (probably 1.0). If you change the doctype to XHTML, you'll get several errors, because some of the element combinations and attributes are not allowed in XHTML.
decide what you want. The new features of HTML5? The rather strict rules of XHTML? Then
, depending on your decision, either ignore the warning. Or change the doctype and fix the errors.
If you're not sure what you want, read up on the differences between the (X)HTML variants and maybe open a new thread "Should I use HTML or XHTML? Which version?". But forget about this stupid warning and don't fumble with the doctype just to get rid of it.