Bug 218996

Safari is the last browser lacking some PerformanceResourceTiming fields:
https://developer.mozilla.org/en-US/docs/web/api/performanceresourcetiming

(see also the following bugs: 168543 198293 185870)

'transferSize' (and 'encodedBodySize') is interesting because for example it allows distinguishing page loads with full cache vs empty cache for performance monitoring:

https://nicj.net/resourcetiming-in-practice/#cached-resources

Based on Nic's blog, I've been using the following code in production to iterate over critical requests and check for cache status and calculate cache hit ratio (and then be able to split performance monitoring graphs based on that):

```
if (window.PerformanceResourceTiming && 'transferSize' in PerformanceResourceTiming.prototype) {
  function getCacheStatus(item) {
    if (item.transferSize === 0) {
      return 'hit'
    } else if (
      item.transferSize > 0 &&
      item.encodedBodySize > 0 &&
      item.transferSize < item.encodedBodySize
    ) {
      return 'revalidation'
    }
    return 'miss'
  }
   
  performance.getEntriesByType('resource').filter(...).map(getCacheStatus).reduce(...)
}
```

But since Safari does not support it, it's not possible to have this kind of visibility about cache hit ratios for Safari users, nor compare cold cache vs warm cache RUM data.