[css-variables] Can custom properties be reset with all?

So, the spec seem to be pretty clear about this:

> Custom properties are not reset by the all property. We may define a
property in the future that resets all variables.

The thing is, I've read comments by several people (including the spec
editor) that specifically mention using `all: initial` for custom property
resetting. For example, in this Github comment
<https://github.com/w3c/webcomponents/issues/300#issuecomment-144551648>
Tab says:

> Sure we can...that's what `all: initial` is for. (Alternately, an
equivalent of all for just custom properties has been proposed; it would be
called --, as in --: initial.) You can then let particular custom
properties thru with --foo: inherit.

In addition, Firefox, Chrome Canary, and Webkit Nightly all reset custom
property definitions when using all.

So I guess in addition to the question I posed in the subject line, is this
a recent change?