Compat Issues: yes

Description:
Visiblity:collapse is implemented very differently in various browsers.
One outstanding issue is what happens to cells that used to span the erased tracks and are now oversized.

Based on 2.1 this is a visual change only, so our questions are:

When collapsing and a portion of the cell is visible, what should occur?
A: The cell's contents are clipped (What Safari and Edge currently do)
B: The cell's contents overflow (What Gecko does)
C: You don't collapse the track since the spanning cell is in the track
D: You make this a layout change and you re-layout the cell in the new constraint space

When you collapse a track do you change the size of the table wrapper box?
Currently Gecko/Edge/Webkit all change the size of the wrapper box

CSS 2.1 Option:
Undefined

Testcases:
http://jsbin.com/zodaderama/edit?html,css,output For collapsing of tracks
https://jsfiddle.net/fa5q3rf6/ For showing re-layout of the table wrapper box

This was discussed at TPAC, but with no conclusion reached. It was mentioned the proposed resolution written above was not about the issue at hand, so I just removed it.

An important thing is that something people wanted to keep visibility:collapse NOT affect track sizing to make it possible for an user to collapse rows/columns without affecting scroll position of things BEFORE the removed track. That being said, since we change the size of the table wrapper, we might move the table itself if it is floated... or affect parent grid, fragmentation, etc... It is not clear what the way forward is in this case.

In case we cannot find an agreement on this, I would note that in additions to previously proposed options, we can also decide to drop the feature or adopt Edge/Webkit behavior, which is the closest you can get from CSSS 2.1.