At present, HTML5 does not mandate support for [ORIGIN] in a compliant UA, which means that a UA need not transmit an Origin header; however, due to the citation of Origin header in  step 5, readers may misunderstand the requirements in this regard;
To avoid misunderstanding, please add an informative note under the last paragraph of step 5 "For the purposes of the Origin header..." as follows (or an equivalent):
Note: A user agent is not required to support [ORIGIN], which means that sending a Origin header is not required by this specification.
This bug was cloned to create bug 17836 as part of operation convergence.
Silvia, this has been fixed in WHATWG revision r7414.
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Rationale: accepted WHATWG change