It's a bit more complicated.
The big standards organisation is W3C. They only call it a standard after everyone agrees on what the standard is and there are implementations in the field that prove that the model works. In that sense they are a bit like the IETF. Part of the IETF motto (TAO): "We believe in rough consensus and running code".
So in the case of HTML5, all browsers will implement the parts of the HTML5 they want to first and only when there are multiple implementations of a feature/part of the HTML5 standard, everyone agrees on what that part of the standard should look like and the documents are ready will W3C rubber stamps it a standard.
So you can already use it before it is a standard. Most parts, by now probably pretty much all of it, of the specification is stable. They are just changing documents to improve working and adding clarifications.
Using the implementations is actually encouraged, because the vendors want to see how it is being used to know if the specification actually works in the way it was intended. Or if it is just to complicated to work with.
Then you have the WHATWG, which is a number of browser vendors (Mozilla, Opera, Microsoft, Webkit/Blink) sitting together creating new HTML5 ideas and standards documents. Those standards documents can eventually be used as a basis by the W3C.
The WHATWG was formed when the W3C, a long time ago, said: all HTML will be XML based in the future. And basically said: HTML is a document format. The WHATWG said: no, way. Let's start a new group of people, because we don't want to deal with strict XML and we actually make it possible to let the web be an application delivery platform.
So really HTML5 pretty much is done. All the browser implementations are done, except for support of certain parts or features.
HTML 5.1 is just a working document title. It is just a set of new features being added to HTML 5 which will end up as part of HTML5.
Fun fact about the picture element is: it did not come out of the WHATWG or W3C or browser vendors, it came out of a community of webdevelopers to create 'responsive images'. A problem that didn't have a good solution yet.
Responsive images is about downloading a the right size of image based on the device it will be displayed on.