PerformanceResourceTiming: renderBlockingStatus property

Render-blocking resources are static files, such as fonts, CSS, and JavaScript that block or delay the browser from rendering page content to the screen. The browser determines these blocking resources automatically and doesn't render any pixel to the screen before all stylesheets and synchronous scripts are loaded and evaluated. This prevents Flash of Unstyled Contents ("FOUC").

In addition to the automatic render-blocking mechanism, blocking="render" can be provided as an attribute and value to <script>, <style> or <link> elements to specify explicit render-blocking. For example:

<script blocking="render" src="important.js" defer></script>

The renderBlockingStatus property can have the following values:

"blocking"

The resource might potentially block rendering.

"non-blocking"

The resource does not block rendering.

The renderBlockingStatus property can be used to see resources that block rendering.

Example using a PerformanceObserver, which notifies of new resource performance entries as they are recorded in the browser's performance timeline. Use the buffered option to access entries from before the observer creation.

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    if (entry.renderBlockingStatus === "blocking") {
      console.log(`${entry.name} is render-blocking.`);
    }
  });
});

observer.observe({ type: "resource", buffered: true });

Example using Performance.getEntriesByType(), which only shows resource performance entries present in the browser's performance timeline at the time you call this method:

const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
  if (entry.renderBlockingStatus === "blocking") {
    console.log(`${entry.name} is render-blocking.`);
  }
});