As the question often comes up about why Bootstrap does not do "true"
ARIA menus (with their role="menu"
etc), add an admittedly lengthy
note (tl;dr because BS is generic, and ARIA menus are specific and quite
limiting).
Additionally, fixes up the <a>
example for dropdown trigger, with the
missing role="button"
and neutering the href
(which would be useless
anyway since BS overrides the link-like nature of the <a>
so it could
never be triggered/followed anyway)