Before cloudflare validate request main domain request 503 status code

I guess this is how Brave reacts to some 4xx and 5xx errors returned from websites.

Refer to this article:

The Brave web browser can now automatically detect when a webpage is unavailable and will offer to search the Wayback Machine for a backup, the Internet Archive has announced. Although the 404 error code is the most well known, the announcement notes that the feature also works for 408, 410, 451, 500, 502, 503, 504, 509, 520, 521, 523, 524, 525, and 526 errors.