`getPropertyValue` can't read `outline`

[tigercosmos]

<style>
  #out {
    outline: .2em solid red;
  }
</style>
<span id="out">
  <span id="in">link</span>
  <span id="in">href</span>
</span>
<script>
  var val = window.getComputedStyle(document.querySelector("#out")).getPropertyValue('outline');
  console.log(val);
</script>

should get rgb(255, 0, 0) solid 2.66667px, but log nothing in Servo.

related code:

servo/components/script/dom/cssstyledeclaration.rs

Lines 228 to 232 in 5683f09

	fn get_property_value(&self, id: PropertyId) -> DOMString {
	if self.readonly {
	// Readonly style declarations are used for getComputedStyle.
	return self.get_computed_style(id);
	}

servo/components/script/dom/cssstyledeclaration.rs

Line 211 in 5683f09

fn get_computed_style(&self, property: PropertyId) -> DOMString {

[emilio]

That's because outline is a shorthand, and we return the empty string for those.

I think we should support shorthands in getComputedStyle and I'm doing related work in FF, but FF also returns the empty string.

[tigercosmos]

I see. I use Chrome to test and didn't find FF not support, too.

[mrobinson]

I'm going to close this as a duplicate of #22838, which seems to have a bit more information.