diff --git a/validator/js/engine/validator.js b/validator/js/engine/validator.js
index 58b51449af3f..35cdec3b43b4 100644
--- a/validator/js/engine/validator.js
+++ b/validator/js/engine/validator.js
@@ -3851,6 +3851,13 @@ function getLayoutSizeDefinedClass() {
   return 'i-amphtml-layout-size-defined';
 }
 
+/**
+ * @return {string}
+ */
+function getLayoutAwaitingSizeClass() {
+  return "i-amphtml-layout-awaiting-size";
+}
+
 /**
  * @param {!generated.AmpLayout.Layout} layout
  * @return {boolean}
@@ -3866,6 +3873,14 @@ function isLayoutSizeDefined(layout) {
       layout === generated.AmpLayout.Layout.RESPONSIVE);
 }
 
+/**
+ * @param {!generated.AmpLayout.Layout} layout
+ * @return {boolean}
+ */
+function isLayoutAwaitingSize(layout) {
+  return layout === generated.AmpLayout.Layout.FLUID;
+}
+
 /**
  * @param {string} layout
  * @return {!generated.AmpLayout.Layout}
@@ -4439,6 +4454,10 @@ function validateSsrLayout(
       // i-amphtml-layout-size-defined
       validInternalClasses[getLayoutSizeDefinedClass()] = 0;
     }
+    if (isLayoutAwaitingSize(layout)) {
+      // i-amphtml-layout-awaiting-size
+      validInternalClasses[getLayoutAwaitingSizeClass()] = 0;
+    }
     const classes = classAttr.split(/[\s+]/);
     for (const classToken of classes) {
       if (googString./*OK*/ startsWith(classToken, 'i-amphtml-') &&