Current and previous versions of WindowEyes software use Alt+0 through Alt+9 to reference "User-defined windows" that outline specific areas of the screen.
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In Opera, to activate a letter accesskey, you hold down Shift + Esc + letter. When using numbers on the other hand, you must press Shift + Esc, let go, and then press the number.
This becomes more problematic as the number keys also have functionality within Opera...
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This means that if you don't press the number soon enough after you've pressed and released Shift + Esc, you will invoke the default number behaviour instead of the element to which the accesskey is assigned.
As users, we could all get our own way, and we don't have to learn how to have keystroke access to everyone's site, we just learn how to do it within our preferred "smart" user agent. As developers, we could work to define relationships and targets rather than keystrokes that may or may not conflict with keystrokes already defined within a user agent or other piece of technology.