Filter/hook to edit html tag at start of page

[adammw]

There currently is no such filter/hook (to my knowledge) that would allow a plugin developer to add tags such as xmlns or xml:lang into the html tag at the start of the page.
This hinders development of plugins which require this functionality.

[Denis-de-Bernardy]

Long before we'd even consider adding any of this, we'll add a wp_header hook immediately after the body tag. And then, we'd wait for a few years so it gets adopted.

Please use an output buffer on the wp or template_redirect hooks in the meanwhile.