[css-anchor-position] could anchor()'s side argument be optional?

[kizu]

After thinking about #10318 (optional anchor-size() arguments) and #10317 (reorderable arguments of anchor()) for a bit, I wonder if we could make the anchor()'s <anchor-side> argument optional as well?

In my experiments, one of the more common things I was doing was inset: anchor(inside). Would be great to have a shortcut like inset: anchor() for cases like this, as well as allow inset: anchor(--foo) with the implied inside as well.

When comparing inside and outside, for any shorthand (inset, inset-block, inset-inline), only the inside makes sense, as outside results in a nonsensical values when applied to those. While separate single-side inset properties could use outside as the default, I do not think it is more useful than the inside, and is not worth the potential confusion and implementation complexity compared to just always using inside.

The only issue I can think of is related to the fallback value: while anchor(--foo, 0px) will make sense, I don't think it is ok to allow doing something like anchor(,0px) if we'd want to omit both the name and side?