Missing “lang” attribute to throw warning, not error

If an HTML document misses a lang attribute on the html element, html-validate complains as such:

error  doctype should be "html"                     doctype-html

This is inconsistent with how the W3C validator handles lang (it issues a warning), and it also seems draconian given that language information may be available on child elements and/or via HTTP header. Even full absence of language metadata may not mean a problem at all.

Could this be reviewed to at least make consistent with the W3C validator?

(Took the liberty to cruise through this, but happy to provide some test cases if useful.)