If you know HTML and CSS a little already, learning HTML 5 and CSS 3 wont introduce a challenge. You will meet a lot of new, nice, powerful and useful features, to say the least, while staying with the same fundamentals that you know already, and offering a standard that will make working with different browsers not painful.
The best editor is usually whatever one you're most comfortable with - at the end of the day HTML and CSS are just plain text, any editor will work. Personally I use Komodo Edit, I've also heard good things about Sublime Text 2 for general editing.
Because I use Eclipse for JEE (e.g. Eclipse Juno version), I use its default editors too. They are just simple text editors that add color highlighting, etc.; e.g. to highlight commands and html elements. No complains about those editors in Eclipse.