Here are a few links I've collected over the last couple years to help work with the new web standards.
Can I use... – this site is invaluable for seeing which browsers support specific features. Type a feature name (like Web Sockets) or a description (like rounded) into the search box and the site updates instantly. The tabular output shows usable features for a given browser version in green, and if vendor prefixes are required. Usage stats display what percentage of users should be able to use a given feature based on the current global browser usage.
HTML 5 differences from HTML 4 – as specification documents go, this page on the W3C is easy to follow and quickly gets to the heart of the matter. The document presents all of the new elements, attributes, and APIs for HTML 5 (as well as pointing out the obsolete and deprecated pieces).
The HTML 5 Test – hit this site with a web browser to see the features it supports. Chrome 25 scores 463/500. IE 10 scores 320/500.
HTML 5 Boilerplate and Initializr – these sites are template generators to give you a starting point for an HTML 5 application. Includes best practices for elements in the <head> of a document, as well as icons for Apple devices and a basic CSS reset. Optionally include libraries like Bootstrap, jQuery, and Modernizr.
HTML 5 Rocks – a great site for articles, tutorials, and the latest HTML 5 news.
The HTML 5 Quiz – A fun diversion for HTML fans. You can also use this game to start random conversations with people in a pub.