HTML5 is not a single integrated thing. It's a collection of extensions to HTML, some of which are widely-implemented and can be used safely, some of which no-one implements yet, and a whole lot in-between. If you try to treat HTML5 as a coherent single development platform and ‘learn it all’ you will have a really difficult time.
The feature set of the web is constantly evolving and there is not one single point of reference. W3Schools (which is nothing to do with W3C incidentally) tries, but it's chock-full of errors. Don't trust what it says as gospel.
You may need to refer to the definitive HTML4, CSS2. DOM Core and DOM HTML specs to make sure. You will also probably want to look at MDC's DOM reference and MSDN's DOM reference for what Firefox and IE support. The HTML5 spec contains a lot of more up-to-date DOM stuff too, as well as the new HTML extensions, but it is a long and unwieldy document, quite hard to use even by the standards of standards documents. Although not nearly as bad as the impenetrable ECMAScript spec. (Thankfully you will probably be familiar with a lot of that already if you're used to working with ActionScript.)
You don't need an SDK or IDE to develop HTML/CSS/JS. You can use an IDE if you like, but I'm quite happy doing everything in my favourite text editor. There are no build/compile steps to worry about, you just save your file and hit reload, job done. Most modern web browsers have a debugger and other development tools either built in (eg IE8) or readily available as extensions (eg Firebug).