Hope this answers your question:
<input type="button" value="Press" onkeydown="doOtherStuff(); return false;">
return false; successfully cancels an event across browsers if called at the end of an event handler attribute in the HTML. This behaviour is not formally specified anywhere as far as I know.
If you instead set an event via an event handler property on the DOM element (e.g. button.onkeydown = function(evt) {...}) or using addEventListener/attachEvent (e.g. button.addEventListener("keydown", function(evt) {...}, false)) then just returning false from that function does not work in every browser and you need to do the returnValue and preventDefault() stuff from my other answer. preventDefault is specified in the DOM 2 spec and is implemented by most mainstream modern browsers. returnValue is IE-specific.