Netscape kept its bookmarks in a file named bookmarks.html (or bookmark.htm in versions for archaic operating systems permitting only 8+3 filenames) in the user profile directory. It was (kinda, sorta) HTML, with a weird doctype specific to Netscape and some offenses against HTML decency like odd nesting of elements. It used some particular combinations of tags and attributes to represent the bookmark structure. Specific items changed over time in different versions as new features were added, but the basic structure remained similar to allow bookmarks to be shared between different browser versions. The format remained in use in early Mozilla browsers before switching to the SQLite-based bookmark database (with automatic backups in a JSON-based format) used in the current Firefox (since version 3.0).
Doctype: <!DOCTYPE NETSCAPE-Bookmark-file-1>
Netscape bookmark file dates, e.g., the ADD_DATE field, are in seconds since January 1, 1970 (see Unix time).