[css-values-5][css-inline-3] Rounding to device pixels

[fantasai]

Following up on #3720... Several situations that were brought up as motivating use cases for env(hairline) were actually about forcing device-pixel rounding. This isn't something we really want authors to be thinking about normally--stylesheets peppered with device-pixel rounding functions would be a nightmare to understand and maintain.

Ideally the browser engine should handle rounding things wherever rounding is the right thing to do, so one question is: are there places where we should be doing device-pixel rounding that we aren't right now?

One place that comes to mind is line-height. If you have a fractional line height, your lines will not have consistent rhythm. Should line-height (and thus the lh unit) round to the nearest device pixel?

[frivoal]

Probably yes, though there's also a question of at what stage the rounding happens. We probably want computed value time, similar to how it happens for border width snapping (and that way it also influences the lh unit), but do we want a second round of rounding to apply to the logical line height / layout bounds as well.