Domain Validation is required and part of the setup of Accelerated Domains. With domain validation, we check if you have control over the domain you want to enable Accelerated Domains for. There are a couple of reasons these checks are being done, but the most important ones are these:
By validating the control over the domain we make sure that the domain that’s ordered can’t be used in attacks on visitors of the domain itself. If we wouldn’t take this step domains ordered for Accelerated Domains might be (ab)used in man in the middle attacks, hosting of fraudulent pages, etc.
Our Accelerated Domains infrastructure is secured in such a way that only domains that are known and validated by us can use it. Validating the domain will ensure that requests for your site get accepted, while other requests for your content will be dropped. If someone does try to access your content through different means than your site they will get a page similar to this: