Why is Gmail blocking CSS in emails?

Gmail doesn't block CSS totally. It still supports inline style. Why doesnt it support style block, I've no idea. To have a good overview of CSS support in various webmail and email clients, here's a good one. http://www.campaignmonitor.com/css/

I am answering it too late but this may help someone here. To be able to view your mail as it is in gmail and other email clients, you need inline style for each tag in your html, but writing inline css for each html tag is time consuming, to save your time use the builtin tool

http://templates.mailchimp.com/resources/inline-css/

Just paste your whole email template and you will get the html document with inline css in each tag. That much easy :)

Note: you can have your class and id in class as it is.