When Happo finds diffs, it's up to you to either accept or reject that diff. The diffs are presented in a Happo report, and the UI looks something like this:
Accepting/Rejecting a report
Diffs are presented on the right, and on the left you have a sidebar with some filters and a "review" panel 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 change shouldn't be merged.
If you have the right CI setup, the accept/reject status will also be posted back to the PR/commit on github.com.
In rare cases you will see an alert message at the top of the compare page informing you about updates made. This is Happo informing you about changes made to internal infrastructure that might affect the screenshots and cause diffs. These could be e.g. browser updates, internal bug fixes, etc. You'll only be alerted about changes that were made in the timespan between when the "before" report and the "after" report were created.