Simple Markdown viewer
Custom sites can be added in the Violentmonkey userscript settings. For custom CSS, see https://greasyfork.org/en/scripts/480759-styles-for-markdown-viewer.
Chromium: Enable "Allow access to file URLs" in the Violentmonkey extension options at chrome://extensions.
Firefox: There is a bug in Firefox that prevents any Markdown viewer from working: https://bugzilla.mozilla.org/show_bug.cgi?id=1319262.
Workaround:
Create ~/.mime.types file with the following content:
type=text/plain exts=md,mkd,mkdn,mdwn,mdown,markdown, desc="Markdown document"
Restart Firefox
UPD: This will be fixed in Firefox 128 (which will be released on July 9th).
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TODO: Describe another Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1267027 / https://github.com/violentmonkey/violentmonkey/issues/173 (summary: the reason the script doesn't work on GitHub).
UPD: Is it also fixed? Need to check after the release of Firefox 128