Cache-Control Extension 



[Cache-Control stale-while-revalidate](https://texonom.com/cache-control-stale-while-revalidate-daee41434c044b1b92cbaf6676474f47)

```typeCache-Control: immutableCache-Control: stale-while-revalidate=<seconds>Cache-Control: stale-if-error=<seconds>```







