[css-backgrounds] allow optional forward slash in `border-*-radius` long hands

[jsnkuhn]

The border-radius shorthand uses a slash to separate corner widths and heights:

border-radius: 2em 1em 4em / 0.5em 3em;

However the slash is not used in the individual border-*-radius long hands. Including the slash gets the property marked as invalid in browsers.

border-top-left-radius:     2em 0.5em;
border-top-right-radius:    1em 3em;
border-bottom-right-radius: 4em 0.5em;
border-bottom-left-radius:  1em 3em;

This is inconsistent and as a result has confused me a number of time. Is there any reason that a slash couldn't be allowed as optional in border-*-radius long hands.

https://drafts.csswg.org/css-backgrounds/#corners

[Loirooriol]

This may reduce some confusion because it's more consistent with the shorthand, but it can also add confusion:

border-top-left-radius: 2em 0.5em;
border-top-left-radius: 2em / 0.5em;

It's not clear that they are the same. Or if you set 2em / 0.5em and then serialize, you would presumably get 2em 0.5em which again may confuse.

Overall I'm leaning towards this not being worth changing.