Note:<head> primarily holds information for machine processing, not human-readability. For human-visible information, like top-level headings and listed authors, see the <header> element.
If the document is an <iframe>srcdoc document, or if title information is available from a higher level protocol (like the subject line in HTML email), zero or more elements of metadata content.
Otherwise, one or more elements of metadata content where exactly one is a <title> element.
Tag omission
The start tag may be omitted if the first thing inside the <head> element is an element. The end tag may be omitted if the first thing following the <head> element is not a space character or a comment.