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.
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.