added box-model example

Author: khan4019

css.html

@@ -213,8 +213,15 @@ <h2>7. specificity </h2>
 <div id="mark">
   <h2>8. box model</h2>
   <p><strong>Question:</strong> What is css box model?</p>
-  <p><strong>Answer:</strong> everything in a web page is a box where you can control size, position, background, etc. Each box/content area is optionally surrounded by padding, border and margin.</p>
+  <p><strong>Answer:</strong> everything in a web page is a box where you can control size, position, background, etc. Each box/content area is optionally surrounded by padding, border and margin. When you set height and widht of an element, you set content height and width</p>
   <p>ref: <a href="http://www.w3.org/TR/CSS21/box.html">W3: box model</a>, <a href="http://css-tricks.com/the-css-box-model/">css box model</a></p>
+  <div>
+    <button id = "boxModel" type="button" class="toggleExample btn btn-primary">show example</button>
+    <div id="boxModelExample" class="hide">
+      <img src="images/box-model.gif" alt="">
+      <p>ref: image taken from google search</p>
+    </div>
+  </div>
 </div>
 <div id="standard_quirks">
   <h2>9. shadow DOM</h2>