[csswg-drafts] [css-transitions]/[css-animations] Proposal: Allow velocity values for transition-duration and animation-duration (#5091)

SetTrend has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-transitions]/[css-animations] Proposal: Allow velocity values for transition-duration and animation-duration ==
Transitions and animations look weird on sibling elements having different sizes.

This is particularly true when, for example, one menu strip only contains a few items while the neighbour menu strip contains many.

See [this JSFiddle](https://jsfiddle.net/hjtyx3ng/1/) for a sample application: The same transition is applied to both menu items, but comparing the left transition to the right it's a rabbit versus a turtle.

That's why I'd like to propose to allow for providing transition and animation duration values in terms of velocity (i.e. length/time).

For instance, valid property values could be:

```css
transition-duration: 1em/185ms;
transition-duration: 2px/100ms;
animation-duration: 5em/1s;
```

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5091 using your GitHub account