Apparently some browsers support the exciting Retry-After HTTP header:

https://tools.ietf.org/html/rfc7231#section-7.1.3

https://stackoverflow.com/questions/3764075/retry-after-http-response-header-does-it-affect-anything

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After

It's not obvious which one do at this stage.