...
The presentation markup of MessageML, called PresentationML, will be re-framed as a strict subset of HTML5, all MessageML documents PresentationML documents will be strictly valid HTML5, although not all valid HTML documents are acceptable MessageML messagesPresentationML messages. These two changes taken together mean that a consumer of messages which is not aware of structured objects can ignore the JSON element of the message and render the MessageML part PresentationML part of the message in isolation and withe semantically valid results.
The API Agent will accept full MessageML, which is a superset of PresentationML and includes a number of convenience markup structures including template macros.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
...