Short answer: a snapshot is created before every change, so everything can be reverted.
In practice, it works like this: before anything is adjusted, we record the current state. If something goes wrong, we restore it. For larger modifications, we first work on a copy of the page so you can review it before it goes live.
The honest disclaimer: just because the technology accepts a change doesn’t mean the page will look good. That is why we also check what the visitor actually sees after every modification. These are two different checks, and both are necessary.