diff --git a/layout/mathml/mathml.css b/layout/mathml/mathml.css
index b146aa6e63b2..e210a0a1aa5e 100644
--- a/layout/mathml/mathml.css
+++ b/layout/mathml/mathml.css
@@ -326,3 +326,8 @@ mscarries {
   -moz-math-display: inline;
 }
 */
+
+/* Do not expose the <mphantom> element to the accessible tree. */
+mphantom {
+    visibility: hidden;
+}