tag-browser: Some small improvements

This patch makes tag-browser layout somewhat usable on mobile.
Additionally it adds few banner hints to use Ctrl keys and basic hint to
select a category when empty.
This also makes the Copy-button hidden until some content is shown in
code-block.

index.html

@@ -18,13 +18,17 @@
   </div>
   <div id="secondaryCategories" class="categoryList">
   </div>
-  <div id="site">
+  <main id="site">
     <div id="targets">
     </div>
+    <div class="banner">
+      <div class="hint keyinfo">Hold <kbd>Ctrl</kbd> or <kbd>Cmd</kbd> to add/remove styles to style set</div>
+      <div class="hint placeholder">Select a style category from the left pane</div>
+    </div>
     <div>
       <code-block data-highlight="css" data-matchlinks="[\w-\.]+\.css -> ./chrome/%s" id="previewBox" class="copy-able"></code-block>
     </div>
-  </div>
+  </main>
 </div>
 </body>
 </html>