Blink and Safari fail WPT by accepting ellipse(10% at 20% 30%)
There should be two radii or none. Authors can use circle() if they only have a single radius.
https://wpt.fyi/results/css/css-shapes/parsing/shape-outside-invalid.html
https://wpt.fyi/results/css/css-masking/parsing/clip-path-invalid.html

https://drafts.csswg.org/css-shapes-1/#funcdef-ellipse
ellipse( <shape-radius>{2}? [ at <position> ]? )

Microsoft reports that usage of ellipse() is low, so we should be able to Intent to Deprecate and Remove (and add a use counter to verify low usage during the deprecation period).
https://developer.microsoft.com/en-us/microsoft-edge/platform/usage/css/clip-path/
https://developer.microsoft.com/en-us/microsoft-edge/platform/usage/css/shape-outside/