I was having issues with 'transitionEnd' not firing, until I realized it had to be 'transitionend'. I know that in the documentation for the TransitionEvent interface shows the events as all lowercase but my brain was typing in camelcase. Upon further looking I noticed all event names are lowercase, but nearly everything else in JavaScript is camelcase.
I took a look at the specs for UIEvent, but couldn't find anything mentioning the requirement for lowercase event names. It just says it's a DOMString and the only reference in that doc to lowercase was in relation to keycode events. Why are the UIEvent names required to be lowercase and where exactly is that specified?