Possible Bug with <div> id element?

Hello everybody.

I am an experienced HTML coder, and I have a technical HTML question that I can't seem to find the answer to.
My page at www.acctadv.com/new_allexceltips.php returns an error because I have <div> tags with numbers for the id.
I use numbers for a DHTML effect of showing and hiding tips as you can see if you see the page.
Is this a bug (not allowing numbers in <div> ids)?
If not, is there a workaround?

Thanks!
Jeremy