Add keyword values for white-space property (#2886)

Author: skyclouds2001

live-examples/css-examples/writing-modes/writing-mode.html

@@ -19,6 +19,20 @@
       <span class="visually-hidden">Copy to Clipboard</span>
     </button>
   </div>
+
+  <div class="example-choice">
+    <pre><code class="language-css">writing-mode: sideways-rl;</code></pre>
+    <button type="button" class="copy hidden" aria-hidden="true">
+      <span class="visually-hidden">Copy to Clipboard</span>
+    </button>
+  </div>
+
+  <div class="example-choice">
+    <pre><code class="language-css">writing-mode: sideways-lr;</code></pre>
+    <button type="button" class="copy hidden" aria-hidden="true">
+      <span class="visually-hidden">Copy to Clipboard</span>
+    </button>
+  </div>
 </section>
 
 <div id="output" class="output large hidden">