<https://adao.co.uk/blog/adding-custom-html-email-signatures-in-osx-mac-mail-v6-5/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://adao.co.uk/blog/adding-custom-html-email-signatures-in-osx-mac-mail-v6-5/>