MaxStale

The "max-stale" request directive indicates that the client is willing to accept a response that has exceeded its freshness lifetime. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its freshness lifetime by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age.

This directive uses the token form of the argument syntax: e.g., 'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate the quoted-string form.

https://tools.ietf.org/html/rfc7234#section-5.2.1.2