I can say with some amount of confidence that the Gmail App on Android does strip the style tag and adds the head section below.
I saw someone allude to this once on twitter - I can't recall who it was so credit to that person if I can find them.
Also, I don't think they'd ever do this but here is what I'd like to see:
First thing to consider is why does MS use Word as a rendering engine? The primary reason I can see is to create a rich experience for their users. You can open Outlook, hit New Message, drag a Power Point presentation and an Excel doc in and hit send. When your coworker opens this message in their Outlook they can dive right into these docs in the message. It's a compelling use case and people are doing this at their jobs all day long.
Similar to how you could tell IE which versions you designed against with Compatibility Mode, we should be able to include a meta tag for render mode.
<meta http-equiv="X-Render-Mode" content="html" />
If this tag is present I know I am sending an HTML email. I am asking Outlook to use a browser control to render this message. I am consciously opting out of the MS Office ecosystem. Sure my messages will lose some functionality - I can't embed an Excel doc in them. I think providing a list of tags / css to support is silly. I want you to support all of them.
The technical challenge in this approach is how to handle when the user hits fwd or reply. Do you convert the message to a Word doc at that point? But surely they could tackle that challenge as opposed to the challenge of getting a Word control to support all the various HTML and CSS that people want to see.
It's in progress. We're overhauling our Android code to be able to update more easily going forward which is what's holding it up a bit. Keep an eye on our blog / twitter where we'll announce when we've updated.
I wrote a small python script to basically do just this. It will build your email from text and html and insert it into any webmail platform.
It's easy cause many designers are on a mac and have python already installed.
We follow the Android market share pretty closely. Unfortunately the uptick for 4.4 hasn't really happened just yet. When it does we'll definitely add support for it.
Regarding: "Gmail appears to provide the original user-agent within the request" - we're not seeing that, does anyone else see how or where exactly in the request that is happening?