[css-contain] content-visibility and SVG

[rwlbuis]

From my current interpretation content-visibility does not apply to most SVG elements since content-visibility applies to
"elements for which [size containment] can apply" (https://drafts.csswg.org/css-contain/#content-visibility)

and (size) contain has a special rule about applying to svg:
"In addition, in the case of [SVG2], the contain property only applies to svg elements that have an associated CSS layout box."
(https://drafts.csswg.org/css-contain/#contain-applies)

So, would that be just outer <svg> and <foreignObject> elements?

I am willing to write some WPT tests for this since I don't think this is actually tested on SVG elements at this time.

[rwlbuis]

I added a test on an SVG element that has no associated CSS layout box (<g>):
web-platform-tests/wpt#42552

[rwlbuis]

@vmpstr what is your interpretation? Also feel free to add more people :)

[vmpstr]

This interpretation makes sense to me. Size containment is important for the optimization that c-v makes, so if it doesn't apply, then c-v should also not apply.

To confirm: the spec would not need to change, since it effectively states this, right?

[rwlbuis]

To confirm: the spec would not need to change, since it effectively states this, right?

Well, one thing I could not find in the specifications is which SVG elements have an associated CSS layout box. Because I kind of know from the implementation to me it sort of is clear, not sure for others.

I realised we do not really test the root <svg>, so I created web-platform-tests/wpt#42594.