Remove mouseclick link focus in menu

[StevenTammen]

Links (and buttons, etc.) need to have :focus styling for accessibility (navigating the site via Tab/Shift Tab; keyboard navigation). However, clicking on a link with the mouse activates the :focus styling, which looks strange in the menu in normal use.

It is surprisingly painful to set things up such that you retain keyboard-based focus styling for accessibility while getting rid of the odd-looking mouseclick focus styling, particularly if you want to do things with only CSS.

See:

• https://alastairc.uk/2016/09/fixing-outlines-on-click/
• https://stackoverflow.com/questions/19053181/how-to-remove-focus-around-buttons-on-click
• https://stackoverflow.com/questions/31402576/enable-focus-only-on-keyboard-use-or-tab-press

Third link especially has good implementation ideas.