I reference these sites a lot in my blog posts and in everyday conversations with other devs so I thought I would put them out here in case someone would find it useful. This isn’t an exhaustive list just what I could think of at the moment and I’ll probably add to it as I find other resources.
Really useful site that lists all of the new stuff in HTML5, whether you can safely use it or not, and links to polyfills if they are needed. Good stuff!
If you ever struggle with layouts, floats, positioning (relative, static, fixed, absolute) or display (inline, block, inline-block) then go through this really well done tutorial. Even if you know this stuff you will probably learn something!
Awesome live demo of newer css features which you can toggle on/off to see how they affect the demo. It also lists browser prefixes if needed and states which versions of which browsers support the feature.
If you want to know if a browser supports a css feature this is the definitive guide.