<content>
is not a valid HTML tag, which is why it's not validating. If you need it for some other purpose, such as finding that block using JS, consider replacing those tags with <div>
and setting a class or id.
To answer your other question, the W3C validator does indeed provide HTML5 validation, although it is listed as experimental. Your current document should be detected as HTML5, but you can expand the More Options
link and select HTML5 as the Document Type
.