Hi Newbwe. Welcome to the forums.
Well, there are many HTML/CSS books out there already, so don't get too hopeful of fame and fortune.
My suggestion would be to focus on some angle of HTML/CSS that would make the book unique—if that's possible. I picked up a book on HTML5 recently (despite having quite a few already) because it is making a name for itself by exposing the failings of HTML5. Nothing like a bit of hot drama to lift sales.