sync-load-shadow.diff

diff --git a/dom/base/nsStyleLinkElement.cpp b/dom/base/nsStyleLinkElement.cpp
index 021b9b6320d8..8ebe471cab21 100644
--- a/dom/base/nsStyleLinkElement.cpp
+++ b/dom/base/nsStyleLinkElement.cpp
@@ -398,16 +398,27 @@ nsStyleLinkElement::DoUpdateStyleSheet(nsIDocument* aOldDocument,
     thisContent->AsElement()->GetAttr(kNameSpaceID_None, nsGkAtoms::integrity,
                                       integrity);
     if (!integrity.IsEmpty()) {
       MOZ_LOG(SRILogHelper::GetSriLog(), mozilla::LogLevel::Debug,
               ("nsStyleLinkElement::DoUpdateStyleSheet, integrity=%s",
                NS_ConvertUTF16toUTF8(integrity).get()));
     }
   }
+
+  if (info.mContent && info.mContent->IsInVideoControlsShadowTree()) {
+    RefPtr<StyleSheet> sheet;
+    nsresult rv =
+      doc->CSSLoader()->LoadSheetSync(info.mURI,
+                                      mozilla::css::eAuthorSheetFeatures,
+                                      /* aUseSystemPrincipal = */ false,
+                                      &sheet);
+    MOZ_ASSERT(NS_SUCCEEDED(rv));
+  }
+
   auto resultOrError = doc->CSSLoader()->LoadStyleLink(*info, aObserver);
   if (resultOrError.isErr()) {
     // Don't propagate LoadStyleLink() errors further than this, since some
     // consumers (e.g. nsXMLContentSink) will completely abort on innocuous
     // things like a stylesheet load being blocked by the security system.
     return Update { };
   }
   return resultOrError;