Cita:
Iniciado por emprear 
en que para html no figure en la lista de atributos y para html5 si, puede considerarse una omisión por parte de w3schools ?
¡Definitivamente! La recomendación original de la W3C si lo tiene entre sus atributos:
http://www.w3.org/TR/html4/struct/links.html#h-12.2
Además, ahí incluyen el propósito original para dicho atributo:
Cita:
Iniciado por http://www.w3.org/TR/html4/struct/links.html#adef-type-A type = content-type [CI]
This attribute gives an advisory hint as to the content type of the content available at the link target address. It allows user agents to opt to use a fallback mechanism rather than fetch the content if they are advised that they will get content in a content type they do not support.
Authors who use this attribute take responsibility to manage the risk that it may become inconsistent with the content available at the link target address.
For the current list of registered content types, please consult [MIMETYPES].