Thankfully, Sjoerd Visscher created the "HTML5 Enabling JavaScript", "the shiv":
<!--[if lt IE 9]>
<script src="https://cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.2/html5shiv.js"></script>
<![endif]-->
The code above is a comment, but versions previous to IE9 will read it (and understand it).
Adding New Elements to HTML
You can add any new element to HTML with a browser trick:
This example adds a new element called <myHero> to HTML, and defines a display style for it:
<!DOCTYPE html>
<html>
<head>
<title>Creating an HTML Element</title>
<script>document.createElement("myHero")</script>
<style>
myHero {
display: block;
padding: 50px;
font-size: 30px;
}
</style>
<!--[if lt IE 9]>
<script src="https://cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.2/html5shiv.js"></script>
<![endif]-->
</head>
<body>
<h1>My First Heading</h1>
<p>My first paragraph.</p>
<myHero>My First Hero</myHero>
</body>
</html>