While working on T174819: Show last 250 changes and period selection dropdowns are not keyboard accessible I noticed that it's not possible to tab through ButtonSelectWidgets. You can see this on the demo page; tab will only focus the already highlighted ButtonSelectWidget.
This is sort of a regression as one used to be able to use arrow keys to navigate between the widgets, see for example OOUI 0.24.4.
Ideally one could use tab to navigate between the ButtonSelectWidgets and press enter to "click" on them.