Using the !important keyword in css is a way to prevent other meddlesome programs from taking liberties to interpret your html/css in a way other than what you want.
For example when.
Learn how next to emails that gmail thinks is important, you'll see a yellow importance marker.
If an email hasn't been marked as important, the marker will be empty.
To see all your emails.
Gmail uses several signals to decide which messages to automatically mark as important, including:
Whom you email, and how often you email them which emails you open which.