Html code to design complex nested list

<!doctype html>
<html>
	<head>
		<title>Example of  Nested List</title>
	</head>
	<body leftmargin="200px" topmargin="50px">
		<h1>Nested List</h1>
		<ol type="I">
			<li>
				Background Skills
				<ol type="A">
					<li>Unix Commands</li>
					<li>Vim Test Editors</li>
				</ol>
			</li>
			<li>
				HTML
				<ol type="A">
					<li>Minimal Page</li>
					<li>Headings</li>
					<li>Elements</li>
					<li>
						Lists
						<ol type="i">
							<li>Unordered</li>
							<li>Ordered</li>
							<li>Definition</li>
							<li>Nested</li>
						</ol>
					</li>
					<li>
						Links
						<ol type="i">
							<li>Absolute</li>
							<li>Relative</li>							
						</ol>
					</li>
					<li>Images</li>
				</ol>
			</li>
			<li>
				CSS
				<ol type="A">
					<li>Anatomy</li>
					<li>Basic Selectors
						<ol type="i">						
							<li>Element</li>
							<li>Class</li>
							<li>ID</li>
							<li>Group</li>
						</ol>
					</li>
					<li>The DOM</li>
					<li>Advanced Selectors</li>
					<li>Box Model</li>
				</ol>
			</li>
			<li>
				Programming
				<ol type="A">
					<li>Python</li>
					<li>JavaScript</li>
				</ol>
			</li>
			<li>
				Database
				<ol type="A">
					<li>Flat File</li>
					<li>Relational</li>
				</ol>
			</li>
		</ol>
	</body>
</html>

Output/ Explanation:

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Html code to design complex nested list