Follow up to #11610 (comment).
Outlines are "special" because they create a path that's external to the original border, more like a stroke.
A concave corner (e.g. a star of scoop shape) with a big outline might end up with very long miters.

Note that currently outline rendering is specified in pretty vague terms.

Proposing (based on suggestion from @smfr) to advise implementations to render a round miter for concave corner-shaped outlines.