While developing this blog, I got problems with the actual font: it uses by default what they call (just discovered) ligature.
Many fonts use this way to merge a pair of chars into one. In my case there where the combo “fi” which always turned into a sort of number “5”.

Just looking around, here is how I got rid of this annoying behavior.

font-variant-ligatures: none;

Also, I got the same result with:

font-feature-settings: 'liga' 0;


More:
MDN font-variant-ligatures
MDN font-feature-settings