User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:25.0) Gecko/20100101 Firefox/25.0 (Beta/Release) Build ID: 20131028112627 Steps to reproduce: Open a file with the following HTML code: <table> <tr> <th></th> <th colspan="2">Tuesday</th> </tr> <tr> <td>8:00</td> <td colspan="2" rowspan="2">-</td> </tr> <tr> <td>9:00</td> </tr> <tr> <td>10:00</td> <td rowspan="2" class="collision-left">2 hour long lesson</td><td rowspan="3" class="collision-right">3 hour long lesson</td> </tr> <tr> <td>11:00</td> </tr> <tr> <td>12:00</td> <td rowspan="2" class="collision-left">-</td> </tr> <tr> <td>13:00</td> <td class="collision-right">-</td> </tr> </table> http://jsfiddle.net/KUks6/ Actual results: The cell with the "3 hour long lesson" did not span across the 3 rows vertically from 10:00 to 12:00 accordingly, but only to 11:00, which is only 2 rows. http://snag.gy/ERRP7.jpg Expected results: The cell with the "3 hour long lesson" should have had a 3 row length height through 10:00 to 12:00 instead of only 2.

Corrections to the report above: The provided HTML code presents the bug only with the CSS code below (the one on the jsfiddle page): table,th,td { border: 2px solid black; } table { border-collapse: collapse; text-align: center; } td.collision-left { border-right-width: 1px; } td.collision-right { border-left-width: 1px; }

Also, the page seems to render just fine when remove a row from it.

The cell spans three rows fine (as you can see by putting a background on it). What you're seeing is that the collapsed border drawing is incorrect.

The rendering is the same in Firefox 4, so it's not a recent regression. We might already have a bug report for this issue.

FWIW, reproducible even back to Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.7.13) Gecko/20060410 Firefox/1.0.8

Yes, collapsed borders have all sorts of issues, with a number of bugs filed on them...

confirm that this has been fixed by Bug 1405929.