The AMP Validator
The AMP Validator allows a web developer to easily identify if the web page doesn't meet the AMP HTML specification.
Adding "#development=1" to the URL of the page instructs the AMP Runtime to run a series of assertions confirming the page's markup meets the AMP HTML Specification. Validation errors are logged to the browser's console when the page is rendered, allowing web developers to easily see how complex changes in web code might impact performance and user experience.
It also allows apps that integrate web content to validate the web page against the specification. This allows an app to make sure the page is fast and mobile-friendly, as pages adhering to the AMP HTML specification are reliably fast.