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.