HTML5 and CSS3 are but extensions and additions to (x)html(4) and css2(.1). If you want to be confident with the newer things, you'll have to invest time in the older ones. Especially HTML I think, since building web apps is a place where you really should use the right tag for the job, or know when it really matters (esp with regards to accessibility).
HTML4->HTML5 != Perl4->Perl5. It's not so much an upgrade, but additions and whatnot.