When you click or tap on an element,
or when you use the tab key on
keyboard to move to it,
an element gets focus.
By default, the browser
adds a blue outline to
an element that is focused.
This can be changed
using the :focus pseudo-class:
Tap on the text input to notice the :focus styles applied. Now use the tab key on your keyboard to move to the button and notice the :focus styles of button applied.