The tag style inside the body has almost identical compatibility with the style tag in the head. Each client offers a different quirk with this, so overall I would recommend using a style in your head if you cannot.
For example, Gmail will strip the style tag no matter where you place it. It is best to constantly test using the Litmus or Email service on Acid.
Good link for CSS compatibility in email https://www.campaignmonitor.com/css/
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