This is better than doing the traditional “window.onload” event, as it can attach multiple event handlers to a single event and they all get called.
This has be known to be a flaky way of encasulating event handling into a function. attachEvent & addEventListener do not work the same way and will cause issues, see here for good explanation http://www.quirksmode.org/blog/archives/2005/08/addevent_consid.html
Ryan is right, this can be flaky and should only be used when you are not going to hit other event issues such as key presses. Else the if should be reversed so the W3C approach comes first. Else when you hit IE9+ which do event handling correctly you force IE into the legacy approach.