Unless you really need pretty tooltips, the simplest route is to use title.
Pretty tooltips are nice, but it’s going to be a painful journey – if you’re not fighting with edge detection, you’re definitely going to be fighting with focus stealing should the tooltip appear while a text field is selected. title tooltips won’t have either problem.
Not saying those issues shouldn’t be addressed in UXP. But you’ll have fewer “bang head against desk” moments if you just use title.