Look up HTML5 features, know if they are ready for use, and if so find out how you should use them – with polyfills, fallbacks or as they are.
The collection is growing.
holmes.css is useful for checking the quality of your code (up to W3C HTML5 standards), nitpicking over ensuring markup is valid and semantic and accessility guidelines are met, and when you are tasked to fix up and debug an old, OLD website. It has a simple implementation and a mostly unobtrusive effect on your page. Not recommended for live enviroments.
(also available as simple bookmarklet)
Many CSS Media Queries boilerplates start with a desktop-specific stylesheet, then add queries and styles for progressively smaller viewports. This means that even the small browsers load desktop layout styles and potentially large assets, even when these are set to display:none;.
'320 and Up' starts with a tiny screen stylesheet that contains only reset, colour and typography styles. Media Queries then load assets and layout styles progressively and only as they’re needed. Think of this as responsible responsive design.
It seems like Apple is using the same sneaky marketing tactics as Microsoft does with its HTML5 demos page - which doesn't really use any HTML5 either. This similarity is hardly surprising considering both companies are pushing a non-Free patent-encumbered codec for HTML5 video.
...designed to take advantage of emerging HTML5 features (including Audio elements), intentionally breaking compatibility with older browser versions and substandard contemporaries such as Internet Explorer. Good for Macintosh. Best in Safari 4, Chrome 4, FireFox 3.5, and Opera 10 in that order.