Validity - Refactoring HTML: Improving the Design of Existing Web Applications - page 127

The key difference is that font is an inline element, whereas center is a block element. Thus, if you need to
insert an extra element to which to attach the CSS rule, it will be a span rather than a div .
Used with the -clean option, Tidy fixes up font tags as it does center tags. It replaces the font tags with span
tags and adds CSS rules to the document's header to indicate the changes. This comes with the same caveats
as it does for center : The names it chooses aren't especially meaningful, and Tidy can't distinguish among
different reasons for different font elements. Furthermore, it may introduce unnecessary span s when the font is
nested directly against an enclosing element. I prefer not to use Tidy for this fix.