Change input type to number.

index.html

@@ -9,7 +9,8 @@
   <p>
     <span style="white-space:nowrap;"><span style="font-size:larger;"
         >&#8731;</span><span id=cube style="text-decoration:overline;">
-      </span>&nbsp;=&nbsp;<input type=text id=cuberoot value="" size=2>
+      </span>&nbsp;=&nbsp;
+      <input type=number id=cuberoot value="" min=10 max=99>
     </span>
   </p>
   <p id=result></p>