Email API v.2 -Content Object Explained

Content Object represents the actual contents of a message.
Content Object supports two properties that are self explanatory:
html that contains the full HTML to be delivered to the recipient and,
text to be rendered by those email clients that can not handle HTML content (today there is a very limited number of text based clients).

content This Object encapsulates the actual message body. Email Plus supports two type of message content HTML and UTF-8 TEXT .

htmlStringThe html content of the email message
Must not be null/blank/empty
textStringOptional, the text content of the email message

Remember that no javascript should be included. Most of email clients will refuse to execute that scripts and most of the email providers will consider java script as a bad content and could decrease your IP reputation and therefor have a negative impact on your deliverability.