When you add a widget to the desktop and hover over it without locking the widgets on the screen then a vertical bar slides out from the side of the widget with several controls on it to manipulate various things to do with the widget. These buttons don't have any tooltips whilst similar looking buttons in krunner do have tooltips. this is true of both Plasma 4.x (the screenshot below) and the latest Neon 5 ISO.

If tooltips were added to the widgets then it would help users to feel comfortable using the widgets - particularly newer users. Here's a screenshot of what i mean:



The HIG says that we should "Use tips to label unlabeled controls and to provide additional information.", is there a reason that these haven't got tooltips?

Well, spotted, they definitely do have to have tooltips! No unlabeled control without a tooltip, ever!
Can you file a bug in plasmashell?

will do

Thanks! I just confirmed it