If you want to include a whole DOM in another page so you did... ( old iframe did too )
Html5 engine are going quirk mode with your html style.
sure you'll have a render, something visible.
but , please, stay strict when you invoke Html 5 with your DOCTYPE declaration.
Html have "opening tag" and "closing tag", and singletons too.
Kindly your program need to be written again ;)
I try :
<!DOCTYPE HTML>
<html>
<head>
</head>
<body>
<button class="abutton">a button</button>
</body>
</html>