basic bootstrap implementation results in blank page

I've tried implementing the basics for bootstrap into an xmb forum templare but all I get is a blank page.

Prior to bootstrap:

<?xml version=\"1.0\" encoding=\"$charset\"?>
<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd\">
<html xmlns=\"http://www.w3.org/1999/xhtml\">

<!-- $versionlong  -->
<!-- Build: $versionbuild -->
<!-- $versioncompany -->
<head>

$baseelement$canonical_link

<title>{$SETTINGS[\'bbname\']} $threadSubject - $versionlong</title>

$css

<script language=\"JavaScript\" type=\"text/javascript\" src=\"./js/header.js\"></script>

</head>

and after bootstrap code:

<!-- ?xml version=\"1.0\" encoding=\"$charset\"? -->
<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd\">
<html xmlns=\"http://www.w3.org/1999/xhtml\">

<!-- $versionlong  -->
<!-- Build: $versionbuild -->
<!-- $versioncompany -->
<head>

<!-- Required meta tags -->
    <meta charset=\"$charset\">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

$baseelement$canonical_link

<title>{$SETTINGS[\'bbname\']} $threadSubject - $versionlong</title>

<!-- $css -->

<script language=\"JavaScript\" type=\"text/javascript\" src=\"./js/header.js\"></script>

<!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" integrity="sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M" crossorigin="anonymous">


</head>

Any clues as to the issue[s] appreciated.