store state as attr instead of class
to be then replaced by upcoming custom states
https://html.spec.whatwg.org/multipage/custom-elements.html#custom-state-pseudo-class
remove function name prefix no longer confusable with JQuery
move animation to css
transition requires visibility instead of display
not toggle/ replace event listeners
no class properties
split toggle function
css: use tagnames instead of class if not div
rename class selected -> current
mark previously selected with a class