FONT tags, H1, and UL Problem

I validated all the pages on www.cleanmycomputer.net a while back but I 
originally had some problems validating pages with H1 and UL tags inside 
of font tags.  I changed the problem pages so that the H1 and UL tags 
were not enclosed in font tags.  I would like to know why this happened 
though and what I can do to fix it.  Here is some example problem code:

   <font face="Verdana">
   <h1>Test header</h1>
   <ul type="square">
   <li> test</li>
   <li> test</li>
   </ul>
   </font>

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