See attachment for minimal test case. The problem seems to be with the red cell exhibiting box-sizing: border-box; when it should be box-sizing: content-box; as per CSS standard. Both cells with padding should be wider than the cells without padding. I've tested in both Safari 4.0.5 (531.22.7) and Chrome 5.0.375.125.
Similar to https://bugs.webkit.org/show_bug.cgi?id=12934 but I believe it's a separate bug.
Created attachment 64098 [details] minimal test case
I am unable to reproduce this bug in Safari 15.6 using attached test case and following are results: 1) Red box -> padded is wider than unpadded and it is the case with other browsers (Chrome Canary 106 and Firefox Nightly 104) as well. 2) In other scenario as well - padded is wider than unpadded and match with other browsers. I think it was fixed along the way and can be marked as "RESOLVED CONFIGURATION CHANGED". If I am testing incorrectly, please retest accordingly. Thanks!