Well, put simply: If you can already program (meaning: You know what an if/for/function is, and have an idea what HTML is, and why it isn't a programming language), you'll use it as a documentation/manual to check it - even though for this kind of stuff I recommend MDN (Mozilla Developer Network, or somethin' like that), because I personally found it more useful.
If you have no idea of programming, or web-dev, it will give you an overview.
If you want to get into web-dev, it would be more suitable to look for video-tutorials about html5, css and javascript, to see what it is like. learn the basics