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If FacadeX is active, the public WordPress sites are sent with the xhtml mime type. The internal sites like the administration panel and the login screen, which have many xhtml syntax errors, are still sent with the standard mime type.
You must ensure that your public sites have valid XHTML, otherwise you will get parsing errors in your browser.

FacadeX

Sends public sites with the XHTML mime type. The internal sites, which have many XHTML errors, are still sent with the HTML mime type.

CommentSafe is a really simple plugin that I’ve written for WordPress. Very simply, it takes a list 
of (X)HTML tags – which you can specify through the admin interface – and does one of three things 
with those tags, when they’re found in your comments:
<ol>
<li>Strip tags (example: <code>A bold choice</code> becomes <code>A bold choice</code> – default)</li>
<li>Convert tags (example: <code>A bold</code> choice becomes <code>A &lt;b&gt;bold&lt;/b&gt; choice</code>)</li>
<li>Remove tags and content (example: <code>A bold choice</code> becomes <code>A choice</code>)</li>
</ol>
The comments are changed before being saved to your WordPress database so that, if you decide to 
uninstall or deactivate CommentSafe, your comments will remain sanitised and safe.
For further information and updates, see <a href="http://abitgone.co.uk/projects/commentsafe" rel="nofollow ugc">http://abitgone.co.uk/projects/commentsafe</a>.

aBitGone CommentSafe

CommentSafe is a very simple plugin which removes (X)HTML tags you specify from your comments.

FacadeX vs aBitGone CommentSafe