[css-ui-3] Add an `outline-offset` to shorthand

[yoksel]

Spec: https://www.w3.org/TR/css-ui-3/#propdef-outline

outline doesn't contain outline-offset and it can be added only with a separate line. It would be nice to have outline-offset here too, something like this: outline: 5px/10px solid green

[frivoal]

If we were introducing outline-offset now, I think I would agree with this proposal.

However, both outline and outline-offset have been out for a while now without having a shorthand / longhand relationship. Due to this, the change you're suggestion is likely to be incompatible with existing content.

If outline-offset becomes longhand of outline, then setting outline would always reset outline-offset, even if no outline-offset specific value is provided. For example, the following code would not set the offset to 5px, because the shorthand would win the cascade and reset it to 0.

.highlight {
  outline-offset: 5px;
  outline: 3px solid red;
}

Since this is different from the current behavior, this is almost certain to break existing pages.

Unless you have some evidence or some reason to believe either than this compatibility problem is not occurring, or that even if it does occur it is not actually an issue, I am afraid it is too late to change this.

[yoksel]

Thank you for the explanation : ) So simple. I didn't think about it.