[web-animations] bounded CSS attributes and SVG properties

Hi list,

Some animatable values have closed bounds (e.g. SVG and CSS width and
height values can't be less than 0). When animating these, we have a choice
to either:
* enforce the boundedness within our animations; or
* enforce the boundedness when applying the result back into CSS or SVG

I propose we choose the latter, so that things like:
new Animation(target, [{width: -50px, composite: add}, {width: 50px,
composite: add}], 1);
modify the element from 50px smaller than current size to 50px larger than
current size.

Some animatable values have open bounds (e.g. CSS column-width values must
be larger than 0).
What should we do when an animation results in a value outside these open
bounds? Currently we're thinking that we should set the value to the
smallest representable value larger than the boundary edge, but this seems
somewhat hacky.

Cheers,
    -Shane