diff --git a/extensions/amp-story-panning-media/0.1/amp-story-panning-media.css b/extensions/amp-story-panning-media/0.1/amp-story-panning-media.css
index 6b32094c648d..d80f25280cd6 100644
--- a/extensions/amp-story-panning-media/0.1/amp-story-panning-media.css
+++ b/extensions/amp-story-panning-media/0.1/amp-story-panning-media.css
@@ -18,6 +18,9 @@ amp-story-panning-media {
 
 amp-story-panning-media amp-img {
   backface-visibility: hidden !important;
+  /* Width explicitly set for mobile chrome, desktop safari & desktop firefox. */
+  width: 100% !important;
+  height: 100% !important;
 }
 
 amp-story-grid-layer[template="fill"] amp-story-panning-media amp-img img {