I'm really pleased that email cannot set a style header and has limited ability to have the email deviate greatly in presentation from other email I receive.
> what makes an email different from a web page, other than that it has been sent to you?
I think it's to do with perceived ownership of the environment in which the information is consumed.
Your website... feel free to style, brand, make pretty or ugly. You can make the experience consistent within your realm.
My inbox... I get to control my workflows, how I consume things, in which order, etc. I choose to make the experience consistent within my realm.
That holds fairly well as a definition for why I prefer chat clients that grant me the ability to make all messages consistent, and that do not allow the sender to dictate terms. It also holds up with things like Netflix, it's their realm they can knock themselves out on their design.
It does seem to be whether the environment it is presented is "yours" or "mine".
One is push, the other is pull. I can choose never to go to a URL again, but I can't choose to never receive spam from the same person from a different address and server.
Also, I think many people do that with AdBlock, so it's not unheard of to block styling on webpages of dubious origin.
If you want to show your styles to Gmail users and you have to inline your CSS in every HTML tags you want to alter, which is very ugly and made the email size unneeded large.
Most every email client supports inline CSS, and in my experience, inlining your CSS for emails is the safest bet to ensure your styling is consistent across all email clients. What you can't inline ever (on email or web) is CSS media queries. IMHO, I don't think media queries are supported broadly enough to use them extensively, though Gmail supporting them is a step in the right direction.
I'm really pleased that email cannot set a style header and has limited ability to have the email deviate greatly in presentation from other email I receive.