Can HTML5 be used for both Windows App Dev and for building web sites

I am mostly a database developer and have a little bit of spare time that I want to use to learn a new programming language.

I am thinking about getting my hands dirty with HTML5 and JS to develop some simple Windows Apps.

At the same time a couple of friends asked me if I could help them knock an easy website together (nothing professional, we are talking here about the local sports team etc.).

Am I right in thinking that HTML5 skills will help me with website development as well? This might be a strange question. But for me, app development and website development doesn't seem the same, yet HTML is a web language, that's why I am confused.

Or is there another "language" that would fit both requirements better, i.e. windows app development and website development?

Many thanks in advance.