I have Span tag. I want to change its behavior to a Div.

Is it possible to do this by simply applying the following CSS class to
the span tag:

.MySpan {display:block}

Thanks,
Miguel

The other is for how it's displayed. This is controllable by CSS, just
as you describe.

Can I include a <div> inside a <span>?

Thanks,
Miguel

Not the easiest thing to learn to read though!  (search for "DTD
introduction" or something)

<!--
    HTML has two basic content models:

        %inline;     character level elements and text strings
        %block;      block-like elements e.g. paragraphs and lists
-->
Actually there's a third one too %flow;, which comprises both %inline;
and %block; elements

<!-- %inline; covers inline or "text-level" elements -->
<!ENTITY % special
   "A | IMG | OBJECT | BR | SCRIPT | MAP | Q | SUB | SUP | SPAN | BDO">

<!ENTITY % inline "#PCDATA | %fontstyle; | %phrase; | %special; |
%formctrl;">

So we see from this that <span> is one member of %inline; and also of
%flow;  (see below)

<!ENTITY % block
     "P | %heading; | %list; | %preformatted; | DL | DIV | NOSCRIPT |
      BLOCKQUOTE | FORM | HR | TABLE | FIELDSET | ADDRESS">

<!ENTITY % flow "%block; | %inline;">

We see here that <div> is one member of %block; and thus of %flow; but
NOT %inline;

<!ELEMENT SPAN - - (%inline;)*  >

<!ELEMENT DIV - - (%flow;)*  >

<span> can contain anything from %inline;
<div> can contain anything from %block; or %inline;

So <div> can contain <span>, but <span> can't contain <div>

A div is a block-line element whereas a span is an in-line element.
You can put an in-line element inside a block-line element, but not the
other way around.

See http://www.cs.tut.fi/~jkorpela/html/nesting.html for an indication
of hierarchy of elements.

That would be odd. Why don't you fix the way the markup is generated.

So do you really want your page work the intended way only when CSS is
enabled?