I have no idea, the enter key triggers only rarely for double/multiple presses (in your setup WebDialog uses the OS-provided version of Internet Explorer).
Try onkeyup, it seems to work reliably.
Also in Windows the onkeyup is trapped when inside a drop down list but only for WebDialog.
The enter key with HTMLDialog is used to open the drop down list or if opened to choose the item from the list. Having said that you can trap the onkeydown when focus is on a drop down list.