It seems that WebKitGTK ignores system wide font settings (/etc/fonts). If I make user-specific font settings (~/.config/fontconfig), e.g. the "Hinted Fonts" part from here, the display problems are gone.
When I start the Gnome session using Xorg, everything is fine:
No other applications seem to have this problem, and even in Evolution everything except the message part of the window looks fine. Selecting a different font does not improve the rendering.
Using Evolution 3.38.2-1 and Gnome Shell 1:3.38.2+22+g3a343a8aa-1, mutter 3.38.2+7+gfbb9a34f2-1
Before I write a bug report: has anybody else seen (and hopefully solved) this?
]]>