Validate actual HTML sent by server, not the parse tree
If I attempt to validate a page containing an un-escaped angle bracket, it will say it is OK. I presume this is because it validates the parse tree with the angle bracket already escaped as an entity. The actual online validator raises an error.
<!DOCTYPE html>
<html><head>
<meta charset="utf-8">
<title>Title</title>
</head><body>
<div>></div>
</body></html>