ctrlKey is in the DOM level 2 spec (http://www.w3.org/TR/2000/REC-DOM-Level-2-Events-20001113/events.html#Events-MouseEvent-ctrlKey), and AFAIK is supported by every browser since Netscape Navigator 4.  Regardless, because of how we're filtering it out, if the property didn't exist in some modern browser, ev.ctrlKey would be undefined, which would enter the tab-switching routine (so the system would still behave as expected).