HTML5 is definately the way to go. It's so much simpler to use than previous versions. For example, I'm making an HTML5 only site that's compatible with ie6 all way way through ie10 (i'm only making sure it's just usable, and not completely borked. If you want full compatibility across the board, you got too much time on your hands) and, of course, the other major browsers. I don't have to worry much about compatibility, because for the most part, it's handled automagically with javascript.
My html5 tools are as follows:
Twitter Bootstrap combined with
HTML5 boilerplate
To warn you, twitter bootstrap requires compiling (you can get precompiled versions but the ones I found were outdated. The compilation process is pretty complex, I just about gave up on it, but got it to work. I'm feeling nice, so
here's a precompiled version. It's compiled from source just a couple weeks ago. You'll have to read the twitter bootstrap documentation to use it.
That's the html5/visual side. For the scripting side I'm still using PHP because that's what I know.
I use
codeigniter though, which makes php actually pleasant to use. If you program in PHP, that's the framework I'd recommend, though it will take a couple weeks to get comfortable with it.
If you're planning on learning a brand new server-side language, I'd recommend instead of PHP investigating Go or Ruby, which are better designed.