For a tutorial/HOWTO approach, we like the w3schools.com site in particular, because it gives you the ability to fiddle with their examples and see the results in real time. They have guides for HTML, CSS, JavaScript and PHP, among others.
For quick reference and looking things up, we recommend MDN, the Mozilla Developer Network. They have references for HTML, CSS, and JavaScript, among others.
The World Wide Web Consortium (W3C) defines many web standards, and they have information about HTML and CSS. The standards documents can be a bit dry, but they are the definitive final word on the subject.