B, an inline element, can't contain H1, a block level element, either. The same goes for FONT. You have to do it like this.
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<h1 align="center"><font face="Times, Times New Roman" size="6" color="#CCFFCC"><b>DERBYSHIRE PRINTS FROM ART OF THE COUNTRY</b></font></h1>
Frankly, I think it looks nices with left aligned text.