Originally Posted by Toxaris
It is not exactly true. It must be XHTML. However, Sigil will convert it to XHTML to certain extent.
It might be slightly confusing because the file extensions aren't changed by Sigil. They will still show as .html but the requirements of XHTML are met...header, proper closing of tags, use of quotes within the tag, and the use of ID like Toxaris mentioned.
The .html extension does not cause the ePub to fail validation AFAIK.