Created attachment 223674 [details]
In the attached content, the underline of the character "日" should not skip.
The underline of the character "本" should skip the slant line of the glyph, but it cross the slant line of the glyph.
I attached reproduced image and drew red circles to wrong points.
Created attachment 223675 [details]
I didn't read the code, but it looked like text-decoration-skip: ink code used horizontal glyph to calculate underline length.
Looks to me that we're running the path intersection algorithm as if the underline were under the character, rather than on the left hand side.
I just restated what Yuki said ^_^
*** This bug has been marked as a duplicate of bug 128145 ***
Reopened: even if bug 128145 is fixed, text-orientation: upright will cause this bad behavior to be shown
Stop-gap band-aid patch until a real fix can be made: https://bugs.webkit.org/show_bug.cgi?id=128718
Created attachment 295079 [details]
/Applications/Safari Technology Preview.app