In brief
The selector allows a style of focus according to the browser's judgment that the indicator should be visible.
Useful to know
:focus-visible matches a focused element when the browser determines that a visible focus designation is needed. This helps, for example, with keyboard navigation. It is not just another name for :focus: the two selectors express different conditions. [1]
Initial button style is button:focus-visible { outline: 3px solid #075985; outline-offset: 3px; }. The color and thickness are exemplary; check them on the real page backgrounds.
Removing outline without a substitute can leave the person without a guide as to which control will be triggered. Browse the Tab and Shift+Tab, page including menus and dialogues. If the component receives focus but does not have a clearly visible state, the problem is not solved with an added selector alone. Contrast, cut from a container, and the location of the indicator are also important.
Sources
For further detail and to check the information: