Conditional comments are left intact, but their inner (insignificant) whitespace is removed.
(e.g. <... disabled="disabled"> → <... disabled>)
(e.g. <p class="foo"> → <p class=foo>)
Valid attributes are: class, id, style, title, lang, dir, event attributes
Currently, only:</html>,</head>,</body>,</option></thead>,</tbody>,</tfoot>, and </tr>
