Every button style you need, with hover, focus, disabled and loading states. Keyboard focus shows a clear ring.
Use one primary action per view.
Small, medium and large share the same radius scale.
Leading or trailing icons, or icon-only with a label for screen readers.
Disabled and loading states keep their size so layouts do not jump.
Attached buttons for related actions, and a split button with a menu.
For the single main action on mobile screens.