mark-up added by wordpress?

At first I didn’t want:

WordPress should correct invalidly nested XHTML automatically

But I notice that items in my sidebar, handcoded by me, only on my homepage, would get polluted with bad mark-up. Checking the “WordPress should correct invalidly nested XHTML automatically” rendered my code as is.

example:

this handcoded side bar item:

<div class="widget about">
		<h3>About</h3>
		<img src="src" />
		<p>paragraph</p>
		<p><a href="/about">continue reading</a></p>
	</div>

Without checking that xhtml box the above gets rendered like this:

<h3><font face="Arial" size="2">About</font></h3>
<font face="Arial" size="2"><img src="src"></font>
<p><font face="Arial" size="2">paragraph</font></p>
<font face="Arial" size="2"></font><p><font face="Arial" size="2"><a href="/about">continue reading</a></font></p>

Shouldn’t unchecking that box leave my code alone?