https://code.sololearn.com/W4Rj89WoaIhW/#html
Try Modifying this code....
in this example i'm using a subfolder for each the .js file and .css, so in the head section i had to put the full path
Step 1: HTML
!DOCTYPE html>
<html>
<head>
<title>DOM Manipulation</title>
<link type="text/css" rel="stylesheet" href="css/main.css">
<script type="text/javascript" src="js/script.js"></script>
</head>
<body>
<section id="main">
<div class="container">
<div id="otherfx">
<h2>Move the mouse over a square</h2><br>
<br>
<br>
<table style="width:100%;height:100px">
<tr style="align-content: center">
<td onmouseover="bgChange(this.style.backgroundColor)"
onmouseout="bgChange('transparent')"
style="background-color:#0000ff">
</td>
<td onmouseover="bgChange(this.style.backgroundColor)"
onmouseout="bgChange('transparent')"
style="background-color:#00ff00">
</td>
<td onmouseover="bgChange(this.style.backgroundColor)"
onmouseout="bgChange('transparent')"
style="background-color:#ff0000">
</td>
</tr>
</table>
<br><br>
</div>
>
</div>
</section>
</body>
</html>