Forward compatibility and custom things

I'm spawning off a discussion that originated about CSS Custom Properties
because I feel like it's improperly focused.

One of the best things about CSS is really it's forward compatible nature.
It required laying down some basic rules and then doing everything possible
to confirm new language features around them.  Great for implementers, OK -
but much more importantly great for authors who can apply a piece of
understanding across the system.

CSS has a way to denote something nonstandard via a vendor prefix, and it
looks pretty much the same whether that thing is a property, a
pseudo-class, a pseudo-element, etc.  That's a huge win for authors I think.

Historically, CSS has been kind of hostile to the idea of author defined
anything- but now we're reconsidering.  I think that there is much sense in
advising on a similar scheme that works everywhere in CSS for author
provided custom things.  Secondarily, it would be even better in my opinion
if one could logically draw sense of the relationship to existing vendor
provided custom things.

The one thing we have far-ish along in the works is Custom Properties and
it doesn't provide an answer but we have been advised by the implementers
that there is a narrow window to align something simple like this.

It seems to me a fast and decisive decision about whether to do or not do
this is not only possible but entirely warranted.