Other resources

Mozilla Developer Network (MDN), http://developer.mozilla.org) provides a large amount of data regarding web development. There are topics such as HTML, JavaScript, CSS, DOM, AJAX, SVG, WebGL, and plenty more. Some of the information may be Firefox specific but most of them are not. Kuma, the platform that powers MDN is available on GitHub, https://github.com/mozilla/kuma. Besides that there are many other ways to contribute towards the improvement of the documentation, see more on https://developer.mozilla.org/en-US/docs/Project:About#About_MDN. At the time of writing, MDN is the most comprehensive and the simplest documentation for everyday web development.

HTML5 rocks (http://www.html5rocks.com) is a great resource for tutorials and articles. The project is run by Google and most of the Chrome team, but there are many other nongooglers joining in. Some of the tutorials are using a lot of Chrome specifics, but they are still one of the best articles out there. The project is available as GitHub repository on https://github.com/html5rocks.

Dive into HTML5 (http://diveintohtml5.info/) by Mark Pilgrim is one of the cult books that is funny and provides great starting point.

HTML5 test (http://html5test.com/) gives a score of your browser, and offers comparison between different major browsers.

More detailed information on support across browsers is available on quirks mode http://www.quirksmode.org/compatibility.html.