website: texinfo macros for both ID and classes

[Paul]

For the website I'm thinking about adding a macro that can create a div 
with both an ID and classes.  This would be good to have in general and 
more intuitive for those used to html.  For example:

  @macro div {ID, CLASSES}
  @html
  <div id="\ID\" class="\CLASSES\">
  @end html
  @end macro

Used like so:

  @div {an-id,first-class second-class}


Currently we have divId and divClass that could be replaced by such a macro.

  @div {an-id}  -->   <div id="an-id" class="">

  @div {,first-class second-class}  -->   <div id="" class="first-class 
second-class">

Unfortunately there seems to be no conditionals in texinfo macros so I 
don't see a way to avoid the empty id="" and class="" in that scenario.  
To avoid that we would have to keep divId and divClass and add 
divIdClass.[0]  But I think having one macro for divs would be simpler 
and easier for contributors, which might be the best variable to 
optimize for.

If we did this for divs it would make sense to do the same for spans and 
our current spanClass.

Thoughts?

[0] Maybe pluralize divIdClasses and divClasses to make it clear that 
you can have more than one.

-Paul