[css-rhythm] Minor example udpate

Author: kojiishi

css-rhythm/examples/snap-height.html

@@ -21,6 +21,10 @@
   snap-height: 18pt;
   line-height-step: 18pt;
 }
+.grid #example {
+  background-size: 100% 18pt;
+  background-image: linear-gradient(to bottom, #00bcd4 1px, transparent 1px);
+}
 .line-height-step #example > * {
   margin: 0;
 }
@@ -38,6 +42,7 @@
   border: thin solid black;
   padding: .5em;
   margin: .5em 0;
+  user-select: none;
 }
 #control {
   -webkit-user-select: none;
@@ -93,6 +98,7 @@
       <label><input type="checkbox" value="use-inline-block">Use inline-block for h2</label>
     </span>
   </div>
+  <label><input type="checkbox" value="grid">Show grid</label>
   <label><input type="checkbox" value="editable">Editable</label>
 </div>
 <div id="example" spellcheck="false">