itemscope in XHTML

xmllint:

file.html: 26: parser error: Specification mandate value for attribute itemscope

https://dev.w3.org/html5/spec-author-view/microdata.html#attr-itemscope says:

The itemscope attribute is a boolean attribute.

https://dev.w3.org/html5/spec-author-view/common-microsyntaxes.html#boolean-attribute says:

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.