Re: 'Section lacks heading' invalid for aside/section/header
Validating the following generates the error 'Section lacks heading'...
it is a 'warning' not an error, it is not a MUST requirement its a SHOULD
It seems more logical to accept the generic 'header' element as a valid header for sidebar section elements.
header  is not a generic heading , it has a different meaning , for example header does not contribute to the document outline, and different accessibility layer semantics <hx> = heading <header> = banner when scoped to <body>.