There was a problem on the search page of the site with a double ID:
<div class="form-item" id="edit-keys-wrapper">
<label for="edit-keys">Enter your keywords: </label>
<div class="container-inline"><div class="form-item" id="edit-keys-wrapper">
<input type="text" maxlength="255" name="keys" id="edit-keys" size="40" value="" class="form-text" />
</div>The ID: `id="edit-keys-wrapper"` comes up two times! This is not W3C allowed.
I've downloaded and installed a clean Drupal 6.28 to be sure it's not a theme error.
I posted this on StackExchange as well and got a fix for it: http://drupal.stackexchange.com/questions/72399/drupal-6-search-module-d...