About W3C and WHATWG

• W3C: World Wide Web Consortium is the organization responsible for defining standards for the web, known as HTMLx (where x is the current version). The current version is HTML5.
• WHATWG: Web Hypertext Application Technology Working Group is the organization that "maintains and evolves" HTML as a language.

Resources

• HTML5 Standard, as published by W3C
• caniuse.com, which keeps track of different capabilities and whether they can be used in different browsers
• Markup Validation Service, by W3C
• Browser Statistics, by W3Schools, which shows current statistics on use of different browsers