I use Pinboard, which doesn't sync to browsers at all, though it does import from them and there are extensions for most(all?) browers. It has excellent tagging capabilities. Every once in a while I switch browsers or do a fresh install of one, over time set up the bookmark toolbar. In theory it would be nice to have that stored in Pinboard, but on the other hand being forced to rethink what I most want easy access to can be a good thing. Having a different set that sync to mobile would be good.
With a monthly fee, Pinboard archives all of the pages you bookmark, so that handles dead links very well, and as we've all learned by now I hope, if we like a service we're usually better off paying for it than suffering ads, or the buyout from some giant corporation that will end or ruin it.
The managing that I would really like to see is the ability to view bookmarks based on tag in a graphical way with overlapping circles (think Venn diagram), and edit the tags on a bookmark simply by dragging it from one part of he overlapping circles to another.