1. What are the differences? I've no idea what the significance of the source is.
2. The one you linked is hella unreadable. OP's table is, due to the color differences, extremely easy to skim, while the one you linked has only very similar black&white x and checkmark symbols, which require closer inspection to differentiate.
The "main" development site for HTML5 is the WHATWG. That's where all the browser implementors convened when they lost faith in the W3C.
The WHATWG doesn't bother with versions, there will be no HTML6, for example. They just continually update the HTML5 spec. It's a living spec.
The W3C kind of tracks the WHATWG progress and tags certain points with version numbers (that's where the "5.1" comes from).
Although they do some development on their own, and sometimes both sides disagree and the specs diverge. At some point WHATWG removed an element that W3C retained or the other way around.
I have no idea if they still diverge. I just discard the W3C when it comes to HTML5 and follow WHATWG.
Well, practically speaking, I follow what the Mozilla Development Network documents. :-)
I love http://caniuse.com but I wish they had more detail. For instance, on the XHR page, I'd like to know which browsers support addEventHandler syntax and which ones require the use of onreadystatechange.
Note, the reason why there some features in 5.1 that are not in 5.0 is that there are some features not implemented yet, 5.0 only includes implemented features.
