The point of the phrase "at user option" is that there are some things that the XML processor really should warn you about; but equally, sometimes you may choose not to listen to these warnings, and you should be able to tell the processor to shut up. Note that this is always a negative option; a behavior which the user can disable, not enable.
Few people know what a modal verb is; I certainly didn't, though Michael Sperberg-McQueen claimed, after the fact, that he did. A "modal verb" is "must" or "may" - the amusing thing about this usage is that an earlier draft just said "verb", but Murata Makoto, a WG member and the chief translator of the XML spec into Japanese, exhibiting an intimidating grasp of English grammar, gently corrected the editors on this point.
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