Event propagation in Javascript
If I have an element (html) nested in another element and both of them have a click handler attached, clicking the inner element executes its click handler and then bubbles up to the parent and executes its click handler. That's how I understand it.
Do events bubble up the DOM tree if there are no events attached that are the same and if so, is it worth putting a event.stopPropagation() at the end of every handler to stop this and speed things up?
Answer
events almost always bubble up unless event.cancelBubble=true is set or event.stopPropagation() is used. You are only aware of it, though, when one of your event handlers gets tripped.
See http://en.wikipedia.org/wiki/DOM_events for a list of events which bubble. (Note: in the table of HTML events, cancelable refers to the effectiveness of event.preventDefault() or return false to cancel the default action, not bubbling)
Also see http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-flow, in particular 1.2.1 Basic Flow to understand the capture phase and bubbling phase of event propagation.
EDIT
http://mark-story.com/posts/view/speed-up-javascript-event-handling-with-event-delegation-and-bubbling suggests there is a performance gain by stopping propagation but provides no data.
http://groups.google.com/group/Google-Web-Toolkit/browse_thread/thread/a9af0aa4216a8046 suggests that browsers should be optimized for bubbling behaviour and says there should be no significant performance difference. Again no data.
http://developer.yahoo.com/performance/rules.html#events provides a good technique for improving event-handling performance, but doesn't directly talk about stopPropagation performance.
Ultimately, you'd have to profile the difference to get a good idea of the benefits on your site.