HTML5/CSS can not contain ul with in ul in Microsoft Web Express

Question

I am trying to make a navigation bar with dropdown list, but when I make unordered list with sublist it is giving me an error stating that the ul can not be contained within a ul in HTML 5.

Here is my code:

<div id ="header">
  <nav>
    <ul>
      <li><a href="index.html">Home</a></li>
      <li><a href="#">Assignments</a></li>
      <ul>
        <li><a href="index.html">Assignment 01</a></li>
        <li><a href="Assignment02.html">Assignment 02</a></li>
        <li><a href="Assignment03.html">Assignment 03</a></li>
      </ul>
      <li><a href="#">Tutorials</a></li>
      <ul>
        <li><a href="Week01.html">Week01</a></li>
        <li><a href="Week02.html">Week02</a></li>
        <li><a href="Week03.html">Week03</a></li>
        <li><a href="Week04.html">Week04</a></li>
        <li><a href="Week05.html">Week05</a></li>
        <li><a href="Week06.html">Week06</a></li>
        <li><a href="Week07.html">Week07</a></li>
        <li><a href="Week08.html">Week08</a></li>
        <li><a href="Week09.html">Week09</a></li>
      </ul>       
      <li><a href="mailto:[email protected]">Contact</a></li>
      <li><a href="#">About</a></li>
    </ul>
  </nav>
</div>

Answer 1

Your sub list must be contained within a list item.

For example

<ul>
  <li>item</li>
  <li>Sublist:
    <ul>
      <li>sublist item</li>
      <li>sublist item</li>
      <li>sublist item</li>
      <li>sublist item</li>
    </ul>
  </li>
  <li>item</li>
</ul>

Here is your code modified so that the sublists are contained within your li tags.

<div id="header">
  <nav>
    <ul>
      <li><a href="index.html">Home</a></li>
      <li><a href="#">Assignments</a>
        <ul>
          <li><a href="index.html">Assignment 01</a></li>
          <li><a href="Assignment02.html">Assignment 02</a></li>
          <li><a href="Assignment03.html">Assignment 03</a></li>
        </ul>
      </li>
      <li><a href="#">Tutorials</a>
        <ul>
          <li><a href="Week01.html">Week01</a></li>
          <li><a href="Week02.html">Week02</a></li>
          <li><a href="Week03.html">Week03</a></li>
          <li><a href="Week04.html">Week04</a></li>
          <li><a href="Week05.html">Week05</a></li>
          <li><a href="Week06.html">Week06</a></li>
          <li><a href="Week07.html">Week07</a></li>
          <li><a href="Week08.html">Week08</a></li>
          <li><a href="Week09.html">Week09</a></li>
        </ul>
      </li>
      <li><a href="mailto:[email protected]">Contact</a></li>
      <li><a href="#">About</a></li>
    </ul>
  </nav>
</div>