In this tutorial, you'll learn how to install the validator, validate local amp pages and website URLs. You can understand the PASS or validation error results using command line.
Step 1 - Install AMP Validator
npm install -g amphtml-validator
Step 2 - Validate Local AMP Html Page
amphtml-validator index.html
if your AMP page is valid, You'll see "PASS"
PASS
if there is an error, the validator will show detailed error messages
12:3 Usage of the !important CSS qualifier is not allowed.