added display vs visiblity

Author: khan4019

css.html

@@ -198,9 +198,10 @@ <h2>5. position</h2>
   <p>ref: <a href="http://www.barelyfitz.com/screencast/html-training/css/positioning/">css positioning in ten steps</a>, <a href="http://css-tricks.com/absolute-relative-fixed-positioining-how-do-they-differ/">css position</a>, <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/position">MDN: position</a></p>
 </div>
 <div id="">
-  <h2>6. hidden vs display</h2>
+  <h2>6. display vs visibility</h2>
   <p><strong>Question:</strong> What are the differences between visibility hidden and display none?</p>
-  <p><strong>Answer:</strong> </p>
+  <p><strong>Answer:</strong> <code>display:none</code> removes the element from the normal layout flow and allow other elements to fill in. <code>visibility:hidden</code> tag is rendered, it takes space in the normal flow but hides it.</p>
+  <p>ref: <a href="http://www.vanseodesign.com/css/visibility-vs-display/">visibility vs Display</a></p>
 </div>
 <div id="http_request">
   <h2>7. specificity </h2>
@@ -256,7 +257,7 @@ <h2>10. transition</h2>
 </div>
 <div id="mtuli_lang">
   <h2>11. pre processor</h2>
-  <p><strong>Question:</strong> What are the reasons to use preprocessor?</p>
+  <p><strong>Question:</strong> Have you used any preprocessor? What are the reasons to use preprocessor?</p>
   <p><strong>Answer:</strong> .</p>
 
   ref: <a href="http://www.frontendjournal.com/most-common-technical-interview-question-for-frontend-developers/">most common interview questions</a>