Convert the fading <div> in the Web page back to a <span>

Convert the fading <div> in the Web page back to a <span>

HTML doesn't allow <div> inside <h1>.  I found before that using a
<span> caused the background not to work.  That seems to be down to CSS2
section 10.6.1, describing inline, non-replaced elements: "The height of
the content area should be based on the font, but this specification
does not specify how."

Happily, we can just define this particular <span> to be a block
element, so that it behaves like a <div> even though it's a <span>.
That works fine, and because we're only using it to get a well-defined
content area it doesn't matter if it's ineffective when the stylesheet
is missing.