[resize-observer] "calculate box size" may not return

[Loirooriol]

https://drafts.csswg.org/resize-observer-1/#calculate-box-size

1. If target is an SVGGraphicsElement that does not have an associated CSS layout box
   [...]
2. If target is not an SVGGraphicsElement or target is an SVGGraphicsElement that has an associated CSS layout box
   [...]
   4. return computedSize.

The 1st branch never returns.