The default font is not proportional, BUT you will be able to set it to whatever you like. There will also be an option to disable syntax highlighting.
Could you tell me what browser you are using?
I think it's caused be the missing fallback `monospace` in that list.
I checked with Chrome and Opera and it's monospace there.
Edit: It's monospace in Firefox desktop too (Linux).
I think it's caused be the missing fallback `monospace` in that list.