Idea Summary
It would be nice having a declarative way for adding declarative tooltips for buttons. When you have icon-only buttons is something pretty useful, as it describes crearly what the button does the first time you are using an app.

Use Case
Simplify creation of tooltips for buttons and other elements, as if it could be done declaratively without custom html, the universal theme could modify its look and feel in the future without any additional actions required. The default value of the tooltip should be the label of the button.
Preferred Solution (Optional)
Add “Tooltip” attribute for buttons at least, with default as button label.