Created attachment 294926 [details]
HTML file that illustrates the problem
When one or more cells in a table are given a class that implies a certain border-bottom style, e.g. "border-bottom: 3px solid #f88;", this style sometimes also affects cells to the right of the selected cell(s). This seems to happen only if the affected row is not the final row in the table.
This is demonstrated in the attached HTML file, which may also be seen here: http://folk.uio.no/jkleiser/test/css-bug.html
This file also lists the browsers/OSes where the bug has been seen. I have not tested this on mobile/small devices.
Created attachment 295651 [details]
This happens because during the collapsed border computation, we ignore the colspan value and pick the before border style for the entire spanning (so the first style wins)
If my original demo page should be unavailable, a slightly updated page may be found here: https://quickhacks.telegr.am/css-bug.html
Safari continues to have this bug in 15.5+