At the top of a compare page you'll find a component that looks like this:
This is Happo asking you to manually review the diffs on the page and either accept or reject them. By accepting, you signal to others that the diffs are expected, or at least okay from a design standpoint. The component will then switch to this:
By rejecting, you signal to others that the diffs aren't okay, and that the author needs to follow up with something.
If you have the right CI setup, the accept/reject status will also be posted back to the PR/commit on github.com.