The HTML5, HTML5.1, WHATWG HTML specs say:

An html element's start tag may be omitted if the first thing inside the html element is not a comment.
An html element's end tag may be omitted if the html element is not immediately followed by a comment.

A head element's start tag may be omitted if the element is empty, or if the first thing inside the head element is an element.
A head element's end tag may be omitted if the head element is not immediately followed by a space character or a comment.

A body element's start tag may be omitted if the element is empty, or if the first thing inside the body element is not a space character or a comment, except if the first thing inside the body element is a meta, link, script, style, or template element.
A body element's end tag may be omitted if the body element is not immediately followed by a comment.

However, that "feature" isn't supported. html / head / body elements don't seem to inserted to DOM by oga if both start and end tags were omitted.