In wp-admin/post.php, load a script.
This overwrites the edit() function of registered blocks.
The new function validates the block's content.
And conditionally displays a notice.
@todo: consider not using this for all blocks.
Some blocks are valid once the back-end sanitizer alters them.
Like the 'Audio' block.
This simply uses the save() function of the block to get the markup.

On saving a post in post.php, make request to the permalink for the post.
And validate that for AMP compatibility.
This applies to the 'classic' editor, not AMP.
But refactoring validatePage() would allow it to be used on Gutenberg.