The first iteration of custom elements was based on mutating the prototype of a custom element object, followed by a callback that gives developers the ability to further mutate the object as needed. Google has shipped this in Chrome, but other browsers have been reluctant to follow. I created a Custom Elements wiki page that summarizes where we are at with the second iteration, which will likely be incompatible with what is out there today. There is a couple of outstanding disputes, but the main one is how exactly a custom element object is to be instantiated from markup (referred to as “Upgrading”).
If you are interested in participating, most of the discussion is happening on firstname.lastname@example.org. There is also some on IRC.