Tools

1. jsbin.com

2. Google HTML5 Outliner

3.  http://jsbin.com/binuxo/1/edit -  This example is a simple document, with a hyperlink that, once clicked, displays the table of contents in an <aside> element in the main <section>

4. the W3C conformance checker

5. Polyfills:

http://caniuse.com/, http://html5please.com/

http://modernizr.com/ 

6.  the Google page about rich snippets and structured data

7. Live Microdata Web site

8. Microdata generators

• http://www.barryko.com/seo/html5-microdata-schema-generator/ 
• http://www.microdatagenerator.com/
• http://schema-creator.org

http://miageprojet2.unice.fr/Intranet_de_Michel_Buffa/Outils_de_validation_HTML5%2F%2Fextensions_de_navigateurs

9. http://www.w3.org/developers/tools

10. http://www.w3.org/2009/cheatsheet/

11. Validation tools

W3C VALIDATOR

The W3C validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc.

CSS VALIDATOR

The CSS validator checks Cascading Style Sheets (CSS) and (X)HTML documents that use CSS stylesheets.

UNICORN

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.

LINK CHECKER

The W3C Link Checker looks for issues in links, anchors and referenced objects in a Web page, CSS style sheet, or recursively on a whole Web site. For best results, it is recommended to first ensure that the documents checked use Valid (X)HTML Markup and CSS.

12. Video encoding software

Handbrake (free, open source) or Super (free).

13. Video download helper

VideoDownloadHelper (for Firefox)

14. Creating subtitles

http://www.universalsubtitles.org, also known as http://www.amara.org.