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Sophisticated Accept header may lead to undesirable response
When atompub/xml is chosen at the validation page (default.html) as the format hint, the validation tool sends request having following Http header
Accpet : /; q=0.2, application/atom+xml, application/xml; q=0.5
This intends to prefer application/atom+xml and application/xml, and to fall back to any(/).
Some sever may not understand how to interpret this accept header, and return error message instead of the desired response.
Consider not to state Accept header, or to use a safer one which will always work.