how to get a node field value in a html.tpl.php template file?
Another way - without edit template and adding code in template.php

function THEME_preprocess_html(&amp;$variables, $hook) {
    if (arg(0) == 'node' &amp;&amp; is_numeric(arg(1))) {
      $node = entity_load_unchanged('node', arg(1));

      if (isset($node -&gt; field_FIELD_NAME['und'][0]['value']) &amp;&amp; $node -&gt; field_FIELD_NAME['und'][0]['value']) {
        $variables['classes_array'][] = $node -&gt; field_FIELD_NAME['und'][0]['value'];
      }
    }
  }


replace THEME with your theme_name and FIELD_NAME with your field_name, clear the cache

edit: I change "$node = entity_load('node', arg(1));" to $node = "entity_load_unchanged('node', arg(1));"