Gmail started basic support for style tags in the head area. Found nothing official yet but you can easily try it yourself.
It seems to ignore class and id selectors but basic element selectors work.
<!doctype html>
<html>
<head>
<style type="text/css">
p{font-family:Tahoma,sans-serif;font-size:12px;margin:0}
</style>
</head>
<body>
<p>Email content here</p>
</body>
</html>
it will create a style tag in its own head area limited to the div containing the mail body
<style>div.m14623dcb877eef15 p{font-family:Tahoma,sans-serif;font-size:12px;margin:0}</style>