In Desktop Board when you hover over a Chart (Doughnut) with a layout containing a Count Cube and Vertical % Ranking, the Count and % are displayed.
However, in Web, the value of the Count is not displayed.
Web version of Board should, where-ever possible, provide the same chart functionality as Desktop version, otherwise there may be reluctance/friction from customers to move to web version, as they might see this as a 'loss of functionality'. As developers, it should not be necessary to build 'workarounds' for functionality that is lost (even if this is temporary).
(visibility of roadmap or release dates for resolution would help)
This has been raised as a ticket on Support Desk, and confirmed as a known limitation.