Which email environments don’t support CSS in the head?
Mainly certain versions of Gmail.
As a webmail client, Gmail has to take some precautions when displaying an email. Like most webmail clients, Gmail uses a preprocessor to strip any code from an email that could be a security threat or interfere with the rendering of the client interface itself.
Once an email is run through the preprocessor, it is passed to the app and rendered by the browser. While different browsers can have their own rendering quirks, most issues that designers run into stem from styles that were stripped by Google’s preprocessing.