MSDN has a lot of reference materials for web development. It contains only IE-specific information which can be either an advantage or a disadvantage depending on what you are looking for. Here are direct links to the HTML/CSS (includes DOM), JScript (Microsoft flavor of ECMAScript aka JavaScript) and VBScript. The documentation is very well written, and is a great source for answers to DOM/JavaScript questions, since they tend to be mostly the same across browsers.
Also, as references go, let's not forget the official specifications for HTML, CSS, and the myriad of other standards specified by W3C.
