On Firefox and Safari, the PICO-8 screen can go beyond the bottom boundary of the div that encloses it. When this happens, the bottom part of the PICO-8 screen can be masked/covered by elements below the player. On Chrome it always renders okay.

This screenshot shows what goes wrong:

This is the HTML template as exported by PICO-8 (0.2.0e). I only changed it to add some text below the player, and changed the background color of the "p8_frame_0" div to red.

The page is accessible here for anyone who wants to quickly try and reproduce it. Whether or not the problem occurs depends on the size of the view port. You may have to resize your browser window.