Gecko allows syntax like matrix(1, 0, 0, 1, 5em, 10%), with <translation-value> instead of <number> for the last two entries (it also accepts <number>).  Likewise for matrix3d(), it allows <translation-value> instead of <number> for the fourth- and third-to-last entries, and <length> instead of <number> for the second-to-last.  The spec doesn't allow this, but some Gecko developers want it to:

(David Baron [:dbaron])
> We should change this in the same release that we unprefix.  (Though I think
> maybe we should try to convince other browsers to change.)

(Matt Woodrow (:mattwoodrow))
> This was definitely an intentional choice to provide flexibility for
> authors, consistency with the translate() functions, and consistency with
> our existing matrix() implementation.
> 
> I'd prefer to see this allowed in the spec than remove it from our code.

https://bugzilla.mozilla.org/show_bug.cgi?id=719054#c1

Currently, the other three engines (IE, WebKit, Opera) don't allow this, but it seems like a possibly useful feature.  Do we want Gecko to remove the feature, or everyone else to add it?  Are other engines interested in adding it?

Cf.: https://www.w3.org/Bugs/Public/show_bug.cgi?id=15431