Outlook 2007-2016 only supports a subset of HTML and CSS.
You can read more here:
https://msdn.microsoft.com/en-us/library/aa338201(v=office.12).aspx
While this information specifically is for Outlook 2007 it remains valid for Outlook 2010-2016.
HTML and CSS support in the Outlook email client
Modified on: Wed, 4 Jan, 2017 at 8:13 PM
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