The book concentrates on new features in HTML5 and CSS3 so it will work best if you have some previous experience in HTML and CSS. However some appendices are provided with introductions to HTML, CSS and JavaScript so that if you're a complete beginner you should still be able to follow along if you're prepared to work at it a little bit more.
Sorry, I didn't answer the second part of your post before. I would say it's best practice to use CSS to achieve stuff whenever CSS is capable of achieving it. That way you take advantage of any built in optimization the browser has for the CSS effect and your stuff will still work even if JavaScript is disabled for some reason.
