Print node tags in html header
The reason your getting html in your output is because your using render(), which is just a wrapper for drupal_render. Per that function's documentation, it "Renders HTML given a structured array tree."

If you want just something like:
Cats, Dogs, Otters

it would probably be better to just pull out those values, ether by looping through the field_tags array and pulling out the value into an array and then imploding your array, or simply looping through and creating a string. So instead of render(), use something like:

$tags = array();
foreach($node-&gt;field_tags['und'] as $tag){
  $tags[] = $tag['value'];
}
$variables['field_tags'] = implode(',', $tags);