Why is Gmail blocking CSS in emails?

Question

I used CSS in my email and sent it out.

When I received the email in Gmail, all the CSS was disabled; however, when I retrieved the email in thunderbird or outlook,

Answer 1

Tags have worked for me, I did have an issue where all my CSS was not working because I missed a ; It was a tedious process but I eventually found it.

At this link #9 from here gave me the initial hint that it may be due to an individual line of CSS.