[css-transforms] Proposal: allow `<length>` values in `scale`

[benface]

For a reason that I'm sure is deeply technical, the scale property, as well as the scale(), scaleX(), and scaleY() transform functions, don't allow length values such as px. Percentages and calc() are allowed however, so it would be a huge unlock to allow length values together with them (Codepen here):

<div style="width: 200px; height: 100px; scale: 50px;">
  I expect this to be scaled down to 50px x 50px (same as if `scale` was set to `0.25 0.5`)
</div>

<div style="width: 200px; height: 100px; scale: calc(100% + 50px);">
  I expect this to be scaled up to 250px x 150px
</div>

<div style="width: 200px; height: 100px; transform: scaleX(calc(100% - 50px)) scaleY(calc(100% + 50px));">
  I expect this to be scaled to 150px x 150px
</div>