Mozilla-based browsers used to (before Firefox 3, I believe) store bookmarks as HTML (bookmarks.html) instead of using an SQLite database. It still uses a single HTML file when exporting or importing bookmarks.
The file it generates has:
<!DOCTYPE NETSCAPE-Bookmark-file-1>A bit similar to static web site generators today, only with endless security vulnerabilities.
It still exists but last updated in 2000. It might be fun to make a Docker container with the right dependencies (if that's still possible) to run it.