I understand em sizes are relative to the parent font size. As discussed in other threads (such as Font-size issues comparing chrome and Firefox), the default font size could be different for different browsers, so one should set it and/or use some reset css stylesheets.
I also understand that different browsers may render things in different size if they have a different zoom size.
Nonetheless, for a given parent font size, 1 em should translate to the same number of px, regardless of the browser.
However, in some cases (e.g. https://openlayers.org/en/latest/examples/custom-controls.html, https://github.com/openlayers/ol3/issues/6194) some elements with the same size in em are translating to a different number of px in Firefox and Chrome.
How is that possible?
Edit: On further testing on the same computer:
- Firefox 50 on Windows and Chrome 55 on Windows 10: map icons are 25px
- Firefox 50 on Linux and Chromium 53 on Linux: map icons are 33px