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.
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.