Currently an assert fires if you style text (possibly underlined, regarding text-decoration-skip: ink, and possibly not regarding simply drawing the text) and you supply a letter that is not contained within the SVG font.
Created attachment 232923 [details]
Comment on attachment 232923 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=232923&action=review
> + // The advances will get all messed up if we do anything other than bail here.
> + result.clear(); // Better to be consistent rather than half-correct
Maybe make the comments a bit less hilarious.