Add @font-face descriptors for overriding font metrics

[xiaochengh]

Following the resolution, this patch adds the following @font-face descriptors to CSS Fonts Level 4:

• ascent-override: normal | <percentage>
• descent-override: normal | <percentage>
• line-gap-override: normal | <percentage>

[xiaochengh]

Updated with all comments addressed. PTAL.

[fantasai]

I believe this would go into Fonts Level 5, though?

[xiaochengh]

I don't have a strong opinion which level it should go into.

@tabatkins Could you advise?

[astearns]

@alshamiri5 please stop spamming our PRs with reviews.

[svgeesus]

I believe this would go into Fonts Level 5, though?

That spec doesn't yet exist and if created, would need to be a delta rather than a copy of fonts 4. Where it goes depends on how rapidly we get to consensus and get implementations. I could be persuaded either way.

I would be fine with merging this in fonts-4. We do after all have one implementation already :)

[svgeesus]

Thanks for incorporating feedback to date. This LGTM.

[svgeesus]

As both editors have approved, I'm going to go ahead and merge this. If it later turns out that this should be moved off to a Fonts 5 we can do that, along with any other featurs that are lagging on consensus or implementer uptake.

@xiaochengh thank you so much for the clear explanations, willingness to incorporate changes, and experimental implementation which all made this feature request super simple to get consensus around!

[xiaochengh]

Thanks to everyone for reviewing!