The admin screens for the Validated URLs and the Validation Errors are implemented in a classic WordPress way™, using post list tables and other traditional admin hooks. The end result is that much of the operations require full page loads, and the interfaces are dated. The UI needs to be revamped using modern WordPress interfaces pioneered with Gutenberg (e.g. using React). In the same way, we should eliminate synchronous validation as much as possible: #2069.

Additionally, more of the management of the validation errors should be surfaced inside of the validation error notices on the edit post screen. Instead of merely indicating the number of validation errors as well as the invalid markup with each block, there should be a way to access full error details and source information from the editor itself; authors should just be able to remove the block (#2285) and/or escalate the validation error to an administrator. Non-administrators should not be able to change the validation error state, and they should not be able to access the compatibility tool screens. In short, redesigning the compatibility tool screens should involve components which can be re-used in the context of the validation warning notices.

As part of this, the site validation functionality needs to be fully implemented in the REST API. Currently we have a bare minimum implemented for Gutenberg integration, the read-only amp_validity field added to the post entity. In addition to that, there needs to be endpoints for:

• Listing all validated URLs.
• Requesting a URL to be validated.
• Listing the validation errors for a given URL.
• Listing validation errors.
• Listing URLs containing a given validation error.
• Changing the validation status for one or more errors.