resources

This is a casual list, rather than being exhaustive.

General

A List Apart
Lots of good articles on lots of subjects. Some current ones may seem a bit esoteric, but the archives are there.

HTML

HTML Spec
This is the whole whatwg specification for HTML5. Lots of dense stuff, but all good. Section 4 lists the elements (tags) of HTML, and their usage.
Validator.nu
An HTML5 validator, checks your HTML for errors. Can use either an address (URL) or file upload.
CSE HTML Validator Pro
Another HTML validator, a web-based version of a stand-alone application for Windows.Can use either an address (URL) or file upload.

CSS

CSS3.info
Maybe a little bit dated, but a nice resource on CSS3, with examples.
CSS3 Media Queries Overview
This site prents a long list of media queries and their result, for whatever device you access it from.