Replying to @AmeliasBrain and @patrick_h_lauke, @w3c, @WHATWG
For stuff that *does* have a perceptible impact, then this can be discussed concretely. Random eg: if we have n UAs with an AT action to get to <main>, and they all only jump to the first <main> leaving the rest inaccessible, you can test that, spec should reflect reality, etc.
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