Override html.tpl.php per node type does not work
you don't need different template for this. 
you can set variable in preprocess function and print it.

function MYTHEME_preprocess_html (&amp;$variables, $hook) 
  $node = menu_get_object();
  $dont_cache = array('some_type');
  if (!empty($node-&gt;nid) &amp;&amp; in_array($node-&gt;type, $dont_cache)) {//check if it is node and if it shouldnt be cached
    $variables['appcache'] = '';
  }
  else {
    $variables['appcache'] = 'manifest = x.appcache';//put your value here
  } 
 }


and then print $appcache in you html.tpl.php