Looks like this produces sensible results for interpolation with 0, though I'm
not really convinced about the results from, let's say, 1px to 2000px in the
attached test-case, I would've expected a linear interpolation from that to go
through normal length interpolation.
css-transforms-1 says:
> Two transform functions with the same name and the same number of arguments
> are interpolated numerically without a former conversion. The calculated
> value will be of the same transform function type with the same number of
> arguments.
>
> Special rules apply to <matrix()>.
Which is what we do... I was going to file a spec issue but turns out that it's
already addressed in css-transforms-2:
https://drafts.csswg.org/css-transforms-2/#interpolation-of-transform-functions
Which says:
> The transform functions <matrix()>, matrix3d() and perspective() get
> converted into 4x4 matrices first and interpolated as defined in section
> Interpolation of Matrices afterwards.
Differential Revision: https://phabricator.services.mozilla.com/D4942
6dcf9b7630