I would like to disagree, a little.  The difference is that the former
explicitly specifies that a My2dPieChart isa 2dpie in the HTML source.  There
might well be a host of tools set up  for working with 2dpie's that work
fine with my2dpiechart's because that relationship is explicit in the
source.
> I'd only note that we're getting (once again) dangerously close to 
> introducing more presentation-orientated features into HTML by 
> specifying the chart type by saying that a table of CLASS="2dpie" is 
> a 2D-Pie chart. Like all the other "hints" regarding presentation, 
> I for one would prefer them to live in stylesheets with a more 
> content orientated value used for the CLASS attribute (say, 
> CLASS="meters.readings.june" or CLASS="building.floor.areas").
I agree, let us instead look at an example like this:
<ADDRESS CLASS="internet">
<ADDRESS CLASS="internet.url">
<ADDRESS CLASS="internet.email">
<ADDRESS CLASS="internet.email.personal">
<ADDRESS CLASS="internet.email.business">
Now, ignoring style sheets altogether, I can do all kinds of interesting
queries on the file.  Style sheets are a particular application of this 
type of subclassing, but not the only one.
 Paul Prescod