Make external iframe render work (#35730)

Fix #35729, #17635, #21098

web_src/css/markup/content.css

@@ -545,6 +545,11 @@ In markup content, we always use bottom margin for all elements */
   margin: 0 0.25em;
 }
 
+.external-render-iframe {
+  width: 100%;
+  height: max(300px, 80vh);
+}
+
 .markup-content-iframe {
   display: block;
   border: none;