Issue summary

If you create a page using the REST call /rest/api/content and include a DOCTYPE declaration in the storage format, it will throw the following error.

    "message": "Error parsing xhtml: Unrecognized XML directive; expected CDATA or comment ('<![CDATA[' or '<!--').\n at [row,col {unknown-source}]: [1,323]",
    "reason": "Bad Request",
    "statusCode": 400

Removing the DOCTYPE declaration allows the page to be created successfully.

Suggestion

Add a method to detect the DOCTYPE declaration if this is passed in the body.storage.value of a REST API call, and remove it so the request is successful.