Hover mouse over two "Jumps". Table cell is moving up and down.
Happens only if there's element in second row with height specified in percents.
Hovering Jump's applies class to them (which is not defined anywhere btw) and unhovering sets empty class. There is also some float involved. Looks like a table handling bug to me.
Originally this testcase had IFrame in second row but it turns out that it's not nessecary (can be any other element with 100% height).
This bug affects gmail. In Compose Mail window, clicking arrow next to the "Check spelling" moves frame up and down.