[CSSOM-view] The One True™ viewport definition?

[yoavweiss]

Hello,

I want to define the "Viewport-Width" Client Hint in HTML, and as part of that, want to define strict processing for it, setting its value to the same one as document.documentElement.clientWidth.
At the same time, clientWidth's definition relies on CSSOM's viewport definition while HTML relies on another viewport definition.

Which on those is the "right" one? Is there a good reason to have multiple definitions to the same concept? Halp! :)

[yoavweiss]

/cc @annevk

[xfq]

Related issue: #505