Ext.Button contains

    buttonSelector : "button:first",
This is tolerated by DomQuery, and so works OK even though DomQuery is documented to require

    buttonSelector : "button:first-child",
But with the next generation of browsers including native DOM selection engines, this tolerance will disappear when DomQuery is upgraded to use native selection where available (Which must happen)

See this thread: http://extjs.com/forum/showthread.php?t=45185