[css-borders-4] Merge border-limit and border-clip-*?

[SebastianZ]

Currently, we have two properties that influence how parts of the border are drawn.
border-limit allows to specify which parts of a border to draw (everything or only specific sides or only the corners).
The border-clip-* properties allow to clip specific parts of the border after drawing it.

That's also outlined as an issue in the spec.

Here are two proposals for doing this: the second one is from GCPM, the first one is an attempt to recast it more readably. The names are terrible, known problem, proposals accepted. There is a problem with conceiving this as clipping: if you have dotted borders, you want whole dots always, not parts of dots. So it should be a drawing limit, not a clip.

As they are covering similar use cases, I suggest we merge the features of border-limit into border-clip-*.

The syntax would then be

normal | sides | corners | [ <length-percentage [0,∞]> | <flex> ]+

Where sides clips the sides up to but not including the corners and corners clips the corners.
Clipping is obviously the opposite of of limiting the drawing, though with the above authors can achieve the same effects as border-limit.

Here are examples for how the syntax above covers the use cases of border-limit:

border-clip: normal = border-limit: all
border-clip: sides = border-limit: corners
border-clip: corners = border-limit: sides
border-clip-top: 100%; border-clip-right: 100%; border-clip-bottom: 100%; = border-limit: left
border-clip: 0 1fr 50% 1fr = border-limit: sides 50%

Even without introducing the sides and corners keywords, authors could achieve the same effect when they know the border radius. E.g.

border-clip: var(--border-radius) 1fr = border-limit: corners
border-clip: 0 var(--border-radius) 1fr = border-limit: sides

If we want to use keywords, we need to discuss their names, because border-clip-top: sides sounds wrong.

The name is still up for bikeshedding, of course. As the spec. issue notes, authors probably don't actually want real clipping when using non-solid border styles. Though simple clipping is presumably much faster than an algorithm that takes the style into account.

Sebastian