Ask HN: Cross-browser bookmark sync options

LastPass is shutting down XMarks on 2018-05-01.

What options exist for people who need to synchronise bookmarks between their browsers (so 'chrome sync' and 'firefox sync' aren't answers) AND don't want to use a web page to do it?

I just want to use the Bookmark Bar in each browser (Chrome, Chromium, and FireFox) on any of my computers, and know changes will propagate within a few minutes.

Are bookmark formats complex, or is it the delta/diff/merge algorithms, or the cost of maintaining different browser extensions, that explains why no genuinely similar solutions seem to exist?

Presumably XMarks source will not be released after they shut the service down.