LibWeb: Implement CSS 'contain' property

Author: https://github.com/Psychpsyo
Commit: 67ed676831
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3296
Reviewed-by: https://github.com/AtkinsSJ ✅

Tests/LibWeb/Ref/input/wpt-import/common/rendering-utils.js

@@ -0,0 +1,19 @@
+"use strict";
+
+/**
+ * Waits until we have at least one frame rendered, regardless of the engine.
+ *
+ * @returns {Promise}
+ */
+function waitForAtLeastOneFrame() {
+  return new Promise(resolve => {
+    // Different web engines work slightly different on this area but waiting
+    // for two requestAnimationFrames() to happen, one after another, should be
+    // sufficient to ensure at least one frame has been generated anywhere.
+    window.requestAnimationFrame(() => {
+      window.requestAnimationFrame(() => {
+        resolve();
+      });
+    });
+  });
+}