<rdar://61192562> Because the text is horizontally centered, and there is a timer, and the digits in the timer don’t have the same width, the text jiggles with each tick of the timer. You can solve this by setting font-variant-numeric:tabular-nums and styling the content with a font that supports the tabular numbers opentype font feature (like the system font).
Created attachment 395296 [details] Patch
Committed r259423: <https://trac.webkit.org/changeset/259423> All reviewed patches have been landed. Closing bug and clearing flags on attachment 395296 [details].