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In the second paragraph of section 4.1, the pronoun 'their' in "their values" is ambiguous. Presumably their refers to the associated origin and base URL mentioned in the previous paragraph, but it also may refer to "each XMLHttpRequest object". I suggest you combine the first and second paragraphs, and perhaps substitute "value of origin and base URL" for "their values".
http://dvcs.w3.org/hg/xhr/rev/063520f74003