The Kendo UI Toolbar widget supports adding attributes to the created DOM elements of items via the various attributes options for items, splitButtons and buttonGroups. I wanted to use this feature for adding custom CSS classes to sub-items of splitButtons. However, this seems to stop the "k-item" and "k-menu-item" classes from being added to the "li" DOM element. The sub-item is no longer clickable. This DOJO demonstrates the behavior.
The problem can be fixed by re-adding those classes manually in addition to the custom CSS class.
I have not tested what happens when custom CSS classes are added to usual items or items in buttonGroups.