Font kerning test does not deliver results on Firefox/Thunderbird/Brave

[jimboolio]

Hello!

It looks like the font-kerning test probably does not function properly since when opening it in Firefox or Brave browser, both of the texts look identical on Fedora 40, but kerning works on for example here: https://www.quackit.com/css/css3/properties/css_font-kerning.cfm

If the test is going to be improved, such font has to be used that it actually shows a difference. Also, if on Firefox the user has changed their default font, the test might not work properly. Even the MDN kerning demo breaks when browser default font is Cantarell instead of the default serif.