In HTML5, you can start the first line with a nice, easy-to-remember doctype:
<!DOCTYPE html>
I consider the above line the greatest improvement in HTML5.
The doctype is a web standard requirements and it tells the browser that the document should be processed using HTML5 standards.
Here is a sample HTML5 template:
<!DOCTYPE html>
<html>
<head lang='en'>
<meta charset='UTF-8'>
<title></title>
</head>
<body>
</body>
</html>
You can always base your HTML5 page on the above template.