It is vitally important
, no matter what medium we are talking about these days - whether Mobile or web, to use good css and html markup. It's pretty much the only way you should be doing things.
CSS Web Design Gallery
Check out my own site which lists valid css and xhtml websites from around the web and then take a look at the 'Source' and you'll see how style and content are completely seperate on all of them!
Once you get in to the swing of it, you'll soon see how easy CSS as a language is to learn because it uses a very simple syntax. When you use it enough, you'll soon see that you will copy a lot across to what ever project you're working on and then edit it to fit. Also, once you've made a few mistakes and learnt the solutions, you'll never forget....like riding a bike really!