Maybe it's different between Mac and Windows, but when I open the bookmarks organizer, there is an "Import and Backup" drop-down that has Export HTML as an option.

At the top of the Bookmarks Manager there are 3 buttons (Organize,Views,Import & Backup) on the toolbar to the right of the Back and Forward button.

The first button is the Organize button.
The second button is the Views button.
The third button with the star is the Import & Backup button.

Screenshot: Deleting bookmarks