Your styles are not wrapped in a style tag; the page has no body; you're also missing closing tags...
You need to do the following:
<!DOCTYPE html>
<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta charset="utf-8" />
<title>SupZone-The Best Online Furniture Store</title>
<style>
#mainpage {height:100%;width:100%;margin:0;}
#mainbody {height:100%;width:90%;margin:auto}
#header {height:60px;width:90%;border:2px solid black;font-size:50px }
#ul {background-color:brown;width:80%;height:40px;margin:auto;text-decoration:none; }
</style>
</head>
<body>
<div id="header">
<ul>
<li><a href="#">Home</a></li>
<li><a href="#">About Us</a></li>
<li><a href="#">Contact us</a></li>
</ul>
<div id="mainpage">
<p>main page goes here</p>
<div id="mainbody">
<p>main body goes here</p>
</div>
</div>
</body>
</html>a
Your code needs a lot of work. It might be easier for you to work with an existing framework. Here is a framework with templates to get you started quickly:
Bootstrap · The world's most popular mobile-first and responsive front-end framework.[
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