Zoom level v pixel/em experiment

Measurement: pixels to show 10em.

1. Vary your browser's text-zoom level.
2. Refresh.
3. Record the zoom level and above measurement.

The tool is simple -- the red bar is 10 em wide and varies with zoom level. Whereas the ruler is 800 pixels wide, but does not change per zoom level. As such, you can measure how much the zoom level effects em -- and from that you can determine how much font-size is decreased/increased per zoom level per browser.