Code:

<ul id="myUL">
    <li>Hit the gym</li>
    <li class="checked">Pay bills</li>
    <li>Meet George</li>
    <li>Buy eggs</li>
    <li>Read a book</li>
    <li>Organize office</li>
</ul>
<style>
    body {
      margin: 0;
      min-width: 250px;
    }
    
    /* Include the padding and border in an element's total width and height */
    * {
      box-sizing: border-box;
    }
    
    /* Remove margins and padding from the list */
    ul {
      margin: 0;
      padding: 0;
    }
    
    /* Style the list items */
    ul li {
      cursor: pointer;
      position: relative;
      padding: 12px 8px 12px 40px;
      background: #eee;
      font-size: 18px;
      transition: 0.2s;
      
      /* make the list items unselectable */
      -webkit-user-select: none;
      -moz-user-select: none;
      -ms-user-select: none;
      user-select: none;
    }
    
    /* Set all odd list items to a different color (zebra-stripes) */
    ul li:nth-child(odd) {
      background: #f9f9f9;
    }
    
    /* Darker background-color on hover */
    ul li:hover {
      background: #ddd;
    }
    
    /* When clicked on, add a background color and strike out text */
    ul li.checked {
      background: #888;
      color: #fff;
      text-decoration: line-through;
    }
    
    /* Add a "checked" mark when clicked on */
    ul li.checked::before {
      content: '';
      position: absolute;
      border-color: #fff;
      border-style: solid;
      border-width: 0 2px 2px 0;
      top: 10px;
      left: 16px;
      transform: rotate(45deg);
      height: 15px;
      width: 7px;
    }
    
    /* Style the close button */
    .close {
      position: absolute;
      right: 0;
      top: 0;
      padding: 12px 16px 12px 16px;
    }
    
    .close:hover {
      background-color: #f44336;
      color: white;
    }
    
    /* Style the header */
    .header {
      background-color: #f44336;
      padding: 30px 40px;
      color: white;
      text-align: center;
    }
    
    /* Clear floats after the header */
    .header:after {
      content: "";
      display: table;
      clear: both;
    }
    
    /* Style the input */
    input {
      border: none;
      width: 75%;
      padding: 10px;
      float: left;
      font-size: 16px;
    }
    
    /* Style the "Add" button */
    .addBtn {
      padding: 10px;
      width: 25%;
      background: #d9d9d9;
      color: #555;
      float: left;
      text-align: center;
      font-size: 16px;
      cursor: pointer;
      transition: 0.3s;
    }
    
    .addBtn:hover {
      background-color: #bbb;
    }
</style>

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