Sites

Html5
Use the HTML5 logo
Can I use …
Compatibility tables for support of HTML5, CSS3, SVG and more in desktop and mobile browsers.
WHATWG
Web Hypertext Application Technology Working Group
HTML5 rocks
A resource for developers looking to put HTML5 to use today, including information on specific features and when to use them in your apps.
HTML Goodies
The ultimate HTML resource.
Bootswatch
Free themes for Bootstrap.
Mr. doob
HTML5 demos from Three.js creator.
Code Guide by @mdo
Standards for developing flexible, durable, and sustainable HTML and CSS.
Web fundamentals
A handbook for best practices (by google)
HTML5 Game Engines
Which HTML5 Game Engine is right for you?

HTML5 Canvas Tutorials

simpl.info
Simplest possible examples of HTML, CSS and JavaScript.
WebGL Fundamentals
WebGL from the ground up.

Frameworks and libs

HTML5 Boilerplate
The web most popular front-end template
Popcorn
HTML5 media framework written in JavaScript for filmmakers, web developers, and anyone who wants to create time-based interactive media on the web.
VexFlow
open-source online music notation rendering API.
CutJS
Lightweight, fast, interactable 2D HTML5 rendering engine for cross-platform game development.
GLSL.io
an open collection of GLSL Transitions.

Web components

Polymer
Building blocks for the web
X-tags
The Custom Elements Polylib
Bosonic
Bosonic Web Component

Articles and other resources

HTML5rocks presentation

One line browser notepad

Adding captions and subtitles to HTML5 video

5 Libraries and APIs for Manipulating HTML5 Audio

Special Characters

Learn HTML (good starting point for newbies)

Web components

HTML imports

HTML’s New Template Tag

Custom Elements
a web components gallery for modern web apps
WebComponents.org
a place to discuss and evolve web component best-practices

Canvas

Particles in canvas

HTML5 Canvas demos by Kevin Roast

WebRTC

Getting started with WebRTC
(html5rocks.com)
WebRTC in the real world: STUN, TURN and signaling
(html5rocks.com)