I don’t know exactly what fonts ChatGPT uses, but it’s definitely not my browser default. Behold the grossness.

As a rule, I reset most web apps to use my default fonts as configured in fontconfig. I threw a quick Stylus style with the following guts:

:root {
    --default-font-family: sans;
    --default-mono-font-family: mono;
}

code {
    font-family: mono !important;
}

(Installable via Stylus. Works for me; might not work for you.)