Try this:
<!DOCTYPE html>
<html lang="en">
<head>
<title>HTML CODE FOR WEBSITE TEMPLATE</title>
</head>
<body>
<?php echo "hello world"; ?>
</body>
</html>
This is HTML5. Remember, the difference isn't between "Mac" and "PC", necessarily. It's between BROWSERS. So you'd want to say something like "it works on Firefox (version 26) but not Safari (version xx.xx)", etc.