Apple - Is there a way to set the *default* monospace font in Safari?

The following information was correct when it was posted, but now only applies to obsolete versions of Safari.

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There are unexposed preferences for changing the default fonts and font sizes:

defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2StandardFontFamily Georgia
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2DefaultFontSize 16
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2FixedFontFamily Menlo
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2DefaultFixedFontSize 14

Another option is to edit the CSS file that defines the default styles (look in Preferences > Advanced > Stylesheet), adding something like this, for example:

@font-face {
   font-family: monospace;
   src: local('Menlo');
}

That rule makes any element that uses font-family: monospace (as well as elements like <pre> and <code> by default) use Menlo. Just change the font name (being careful to keep the enclosing quotes) to whatever you prefer.

As of Safari 12, all options for doing so have been removed:

• The Appearance panel is (long) gone
• The defaults keys are no longer respected
• CSS @font-face definitions are limited to a built-in whitelist of local() values, ignoring any other fonts installed on the system, which is part of a broad package of anti-fingerprinting measures

Taken together, this means there is no way to change the defaults away from Times for serif, Helvetica for sans-serif, and Courier for monospace.

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The only thing you can still do is create a user stylesheet to style various elements directly. So for example you could change the default font family from serif to sans-serif:

body { font-family: sans-serif }

Or you can even still name specific locally installed non-system fonts:

body { font-family: 'Open Sans' }

Unfortunately while this works reasonably well for changing the default document font, it is complicated to use the same approach to restyle every element that defaults to a monospace font. I am not sure CSS can even fully emulate the default behaviour for such elements at all – whereas it was previously easy to do so by using either defaults or a user stylesheet with a @font-face rule for monospace (which simply redefined what monospace meant without touching the default styling itself, and so allowed the default behaviour to apply to that other font).

I viscerally dislike reading large amounts of text set in Courier, so it looks like in the future I will have to use another browser to read RFCs…