I was struggling to get quicklook previews of markdown documents in macOS Catalina to work due to the new enhanced security checking. The various open source and self-install solutions out there all failed, even after following all kinds of convoluted procedures.
The solution was PreviewMarkdown which installs via the app store and satisfies all of Apple's new security requirements. It costs $1 which is worth the no-hassle install.
Mac users: the first public beta of Omniweb 5 is out and first signs are good. Lots of cool stuff including workspaces (fantastic), rss support, shared bookmarks, popup trap, loads of useful little extras for web developers, and much more… The version of webcore used is a bit behind Safari feature-wise, but that’s a small detail compared to all the rest.