Is there any way to temporarily allow Safari to load a site with an untrusted website certificate in Safari/High Sierra?
This was possible in Sierra. I marked several roots Untrusted due to their bad behavior in issuing certificates. At one time I think there was an option in KeyChain Viewer to "Ask User", but that was depreciated.
A similar question was answered here (How do I bypass "this connection is not private" warnings in safari?), but the answer is not correct for High Sierra: there is no longer an option "visit the site".