I was working on enhancing a bit my responsive dropdown menu by adding the CSS
:focus-within pseudo-class in order to tab through the dropdown menu items.
To avoid duplication, I thought it is a smart move to group this new fancy CSS
:focus-within pseudo-class with an existing selector. It was beautiful and it looked a lot like a progressive enhancement but in the end, it broke the entire CSS rule within browsers that do not support the CSS
:focus-within, e.g. IE.
If just one of these selectors were invalid, the entire group of selectors would be invalid.