Add support tables for document.documentElement.clientWidth

[wpgbrown]

I think that it would be useful to see the support for:

document.documentElement.clientWidth

[lemondreamtobe]

I agree too!!!! document.documentElement.clientWidth will be null in some environment

[inspiredlabs]

Knowing the status of clientWidth & clientHeight would be useful. The adoption of ResizeObserver has been stale as far as I can tell (and there's tonnes of ways to go about something that's this rudimentry).