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+The default viewer is built on the display layer and is the UI for PDF viewer in Firefox and the other browser extensions within the project. It can be a good starting point for building your own viewer. <em>However, we do ask if you plan to embed the viewer in your own site, that it not just be an unmodified version. Please re-skin it or build upon it.</em>
+
+### Initialization promise
+For custom deployment it is often necessary to know when the viewer is fully initialized. Previously this was possible by listening to the `localized` event, but pull request #11607 implemented a simpler way to do this:
+
+```
+document.addEventListener("webviewerloaded", function() {
+  PDFViewerApplication.initializedPromise.then(function() {
+    // The viewer has now been initialized.
+  })
+});
+```
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