<bmx-tooltip>
A short description of the thing beside it, shown on hover and on focus.
Hover one and then move the pointer into the tooltip — it stays, which WCAG 2.2 SC 1.4.13 requires and most
hand-rolled tooltips do not do. Press Escape while it is open and it goes without the pointer moving, and
does not spring back until the pointer has been somewhere else. Tab to a button and the tooltip opens at once: a
keyboard user has arrived deliberately and should not be made to wait.
The description is on the button before any of that happens.
aria-describedby is written once and left
there, because accessible-description computation reads through a hidden element on purpose — so a screen-reader
user hears it on arrival rather than only if their pointer happens to rest somewhere first.
Every property, event and method: https://docs.binarymission.co.uk/webcomponents/bmx/bmx-tooltip.html