Problem with <HR> tags

I use a program called GEDPAGE to generate genealogical charts as HTML pages. When I run TIDY on them I get the following error message and the <hr> tags are rendered unintelligible.

line 13 column 1 - Warning: unescaped <hr> in pre content

Can anyone please tell me why?

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Graham F Coward
Victoria
Australia
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Visit the COWARD Family History Site at www.alphalink.com.au/~coward/