If your HTML Email contains a CSS comment and inside that comment is a tag (see tag below: <code _tmplitem="277" >) then the email will render empty in Hotmail ISP including Outlook.com!!
/* This most probably won\'t work in all email clients. Don\'t include <code _tmplitem="277" > blocks in email. */
How To Embed Videos in an HTML Email
It should be noted that nowadays most all webmail clients, like Gmail, it is enough to simply put a YouTube link in the body of the email, and Gmail will render that link as a playable video embedded directly within the body of the email. There are other email clients that do this as well. Another way to embed a video is using the HTML 5 video tag, as shown in the following section.
Using the HTML 5 Video Tag
Following is a simple example of how to use the HTML 5 video tag that you would place anywhere within the <body> tag of your HTML email.
Note: this tag only has limited support, in a limited set of mail clients