1. The head element represents a collection of metadata for the document
  2. One or more elements of metadata content, of which exactly one is a title element is required in almost all cases.
  3. HTML5-compliant browsers automatically create a head element if its tags are omitted in the markup
  4. If the document is an iframe srcdoc document or if title information is available from a higher-level protocol (such as email), head may contain zero or more elements of metadata content (W3C)
  5. The title element is a required child in most situations, but when a higher-level protocol provides title information, e.g. in the Subject line of an e-mail when HTML is used as an e-mail authoring format, the title element can be omitted.