When border-collapse: collapse; is applied to a table with borders and a hidden cell, all browsers have issues:

Firefox hides border in the table entirely if all the cells in the row are hidden but the row element is visible. This can be resolved by hiding the row element.

Internet Explorer renders the border of the row that has all it's cells hidden. This can be resolved by hiding the row element.

Opera doesn't render all of the top border of a table if any of the top row cells is hidden and there is atleast one visible cell on the row.

Safari and iCab don't render the td border at all if a cell is hidden.

Test page Workaround is included
Reported by: Merri.

Firefox and Opera preserve the space of the column. The table column should not be rendered, subsequent columns should move over to fill the space as though the column is not there.

Test page Workaround is not included
Reported by: Marc Pacheco.

Opera incorrectly places borders separating the thead and tbody sections.

Safari incorrectly places borders separating the thead and tbody sections it also draws a border around every column contained in the colgroup not just the colgroup.

(ppknote: Explorer doesn't do anything)

Test page Workaround is not included
Reported by: Marc Pacheco.

IE6 throws errors when an element is cloned from one frame and appended to another. This includes pop-ups.

The test page shows two frames and buttons which trigger various combinations of cloning from (hidden) elements in a frame and appending to another.

In Firefox 1.5.09, all permutations work. In Safari 1.3.2, all permutations work at some times, but silent crashes occur at others

IN IE6win, an unappended clone can be held in a variable in another frame (eg. its parent), but it must be appended to the frame from whence it came.

I think this means that DOMloading in pop-ups must be created afresh OR cloned from elements in the pop-up itself.

Test page Workaround is not included
Reported by: Andy Taylor.

Repositioned content via JavaScript coupled with floats is triggering a variety of bugs in various browsers.

Test page Workaround is not included
Reported by: Chris Hester.

In Safari, if you have a form that's inside a div with the clearfix (read about it here) class applied, the form will not show up on the page unless you specifically apply display: block to the containing div.

Test page Workaround is included
Reported by: Amber Rhea.

In Safari and iCab, text-transform: uppercase does not work when applied to the :first-line pseudo-element.

Test page Workaround is not included
Reported by: Beckley Roberts.

If an image is given various heights, but not a width, browsers enlarge it to fit the ratio of the physical dimensions of the image. But over a certain size, bugs can occur.

Test page Workaround is not included
Reported by: Chris Hester.

Adding disabled attributes to <BUTTON> elements in Safari does not have the desired effect.

Disabled buttons with text content are not greyed out and still respond visually to clicks (albeit without generating any onclick events).

In <BUTTON> elements with image content, the addition of a disabled atribute has no effect whatsoever.

Test page Workaround is included
Reported by: Philip Ronan.

Setting multiple interval timers to run simultaneously causes the timers to become completely unreliable in Mozilla/Firefox and iCab.

In most browsers there is a slight variance between the requested timeout and the actual timeout (a few miliseconds per one second timeout). This is normal.

In Mozilla/Firefox, using multiple interval timers causes the intervals to be far too short (a fraction of the requested time), followed by one extremely long interval to make up for the short ones. This pattern then repeats.

Test page Workaround is included
Reported by: TarquinWJ.