In Preferences → “Fonts & Style” → “Monospace” list only fonts satisfying “:spacing=100”
On my system
fc-match -s monospace shows that monospaced fonts are installed. But https://tools.ietf.org/html/rfc3744 is not showed in monospaced font, while on another system Firefox displays it mono-spaced.
Submitting bugs at https://bugs.horde.org asks eventually an ASCII-quiz, which cannot be answered, because the displayed text is not monospace.
How to figure out, why Epiphany 18.104.22.168 or WebkitGTK 2.20.3 do not render monospace fonts as such?