The preprocessor/sanitizer can be an expensive series of operations. If we take a hash of the pre-sanitized HTML and use this as the cache key for caching the sanitized output, it would be possible to short-circuit the entire sanitization process by serving the cache output directly. This would not preclude full page caches since PHP would still be generating the buffered output, but it would be a good way to augment full page caches. Props @ThierryA for the idea.