addMarkers() tries to keep the marker list sorted by start offset:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp?q=documentmarkercontroller&sq=package:chromium&l=749

But removeMarkers() assumes the list is sorted by end offset:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/editing/markers/DocumentMarkerController.cpp?q=documentmarkercontroller&sq=package:chromium&l=379

Perhaps this has been working with the ways markers are currently used, it certainly does not work properly if the markers can be nested.

I think we should just remove all the code that tries to keep this list sorted or assumes it's sorted, I'm not convinced that this is a worthwhile optimization and it seems to be buggy.