Explainer for Render Blocking Status

Explainers/Render_Blocking_Status.md

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+# Render Blocking Status
+
+Proposal to add a new field `render blocking status` to PerformanceResourceTiming which holds a string highlighting the status of stylesheets and scripts.
+
+## Use cases
+
+RUM providers could then easily determine the render blocking resources without having to rely on complex heurestics to determine the same.
+This would enable analysis for below scenaarios and possibly more.
+- Determine resources downloaded before FCP but were not render blocking? (and hence, could've been delayed)
+- Determine resources that were render blocking, but weren't discovered early enough? (and hence, could've benefited from being preloaded)
+(Reference : https://github.com/w3c/resource-timing/issues/262)
+
+
+## API Changes and Example Code
+
+The PerformanceResourceTiming Interface in <a href="https://w3c.github.io/resource-timing/#sec-performanceresourcetiming">resource-timing</a> would be updated to 
+```bash
+[Exposed=(Window,Worker)]
+interface PerformanceResourceTiming : PerformanceEntry {
+    ...
+    ...
+    readonly attribute DOMString renderBlockingStatus;
+    ...
+    ...
+    [Default] object toJSON();
+};
+```
+
+Sample usage:
+```javascript
+const entry_list = performance.getEntriesByType("resource");
+console.log(entry_list[0].renderBlockingStatus);
+```
+
+
+## Render Blocking Status Values
+
+The following values are proposed for the resource blocking status
+
+`blocking` - A potentially render blocking resource
+
+`non-blocking` - Non blocking resource
+
+We primarily reuse the value of the [render blocking](https://fetch.spec.whatwg.org/#request-render-blocking) boolean associated with fetch [request](https://fetch.spec.whatwg.org/#concept-request)
+
+
+## Potential Spec Changes
+
+Fetch ([whatwg/fetch#1449](https://github.com/whatwg/fetch/pull/1449))
+- New boolean field render-blocking added to [fetch timing info](https://fetch.spec.whatwg.org/#fetch-timing-info)
+- [Fetching](https://fetch.spec.whatwg.org/#fetching) is modified to set [fetch timing info](https://fetch.spec.whatwg.org/#fetch-timing-info)'s render-blocking field using request's [render-blocking](https://fetch.spec.whatwg.org/#request-render-blocking)
+
+Resource Timing Level 2 ([w3c/resource-timing#327](https://github.com/w3c/resource-timing/pull/327))
+- [4.3](https://w3c.github.io/resource-timing/#sec-performanceresourcetiming) : Adding new field to interface : renderBlockingStatus
+- Getter steps for `renderBlockingStatus` return `"blocking"` if [timing-info](https://w3c.github.io/resource-timing/#dfn-timing-info)'s newly added render-blocking field is `true`, else `"non-blocking"`
+
+## Changelog
+- Update 1 - Spec changes modified to reuse `Request`'s `render-blocking` boolean.