Here is a specific post to share three useful resources provided by the W3C.
The W3C cheatsheet provides quick access to useful information from a variety of specifications published by W3C. It aims at giving in a very compact and mobile-friendly format a compilation of useful knowledge extracted from W3C specifications, completed by summaries of guidelines developed at W3C, in particular the WCAG2 accessibility guidelines, the Mobile Web Best Practices, and a number of internationalization tips.
HTML5 differences from HTML4 (December 2014)
The Unicorn validator from the W3C
Unicorn is W3C’s unified validator, which helps people improve the quality of their Web pages by performing a variety of checks. Unicorn gathers the results of the popular HTML and CSS validators, as well as other useful services, such as Internationalization, RSS/Atom feeds and http headers.