Add a test to cover using ResizeObserver on SVG elements

[w1ndy]

This PR adds a test to see if ResizeObserver correctly fires events when the SVG elements themselves are resized. All major browsers I've tested failed this test, but spec seems to support this usage (see https://www.w3.org/TR/resize-observer/#calculate-box-size and w3c/csswg-drafts#4032 (comment)).

[atotic]

I think this is behaving as intended. According to spec, RO observers svg's bounding box getBBox(), which does not change when you change the height attribute.

[w1ndy]

I think this is behaving as intended. According to spec, RO observers svg's bounding box getBBox(), which does not change when you change the height attribute.

Thanks for pointing this out. So it looks like this resolution haven't made its way into the spec yet (honestly I wonder if it's ever going to happen...)

[atotic]

If you update the spec, this test will be useful. It would be useful to expand it to cover a few more cases.

In the spec discussion, the elements with layout box were defined as svg:root, *:not(svg|*) > svg, svg|foreignObject > svg.

[w1ndy]

If you update the spec, this test will be useful. It would be useful to expand it to cover a few more cases.

In the spec discussion, the elements with layout box were defined as svg:root, *:not(svg|*) > svg, svg|foreignObject > svg.

Yeah, I've submitted a PR to update the spec. This test would be useful after that PR gets merged (and probably we need to wait until it becomes recommendation?). I can work on this PR to add a few more tests to cover the additional SVG elements with layout box.