how to select one legend element in javascript code example

Example 1: html legend

The HTML <legend> element represents a title caption for the content of 
its parent <fieldset>.

Example 2: legend in html

#legend in html
<ul class="legend">
    <li><span class="greendot"></span>Text</li><br></br>
    <li><span class="yellowdot"></span>Text</li><br></br>
    <li><span class="reddot"></span>Text</li><br></br>
    <li><span class="blackdot"></span>Text</li><br></br>
</ul>
<style>
  .legend { list-style: none; margin-left:10px;}
    .legend li { float: left; margin-right: 15px;}
    .legend span { border: 1px solid #ccc; float: left; width: 10px; height: 12px; margin: 2px;}
    .legend {
             position: fixed;
             top: 62%;
             left: 10%; 
  /* bring your own prefixes */
             transform: translate(-50%, -50%);
            }
    /* your colors */
    .legend .greendot { background-color: #00ff00;}
    .legend .yellowdot { background-color: #ffff33; }
    .legend .reddot { background-color: #ff3333; }
    .legend .blackdot{ background-color: #000000; }
</style>