Does HTML understand "if-else"?

HTML has a limited set of "if/else" test on the browser's capabilities, e.g. whether it can handle scripts (<noscript>), frames (<noframes>), etc.

Your case on Flash-plugins can be handled by fallback content of <object>, for example:

<object type="application/x-shockwave-flash" data="x.swf" width="400" height="300">
   <param name="movie" value="x.swf" />
   <p>Your browser does not support Flash etc etc etc.</p>
</object>

(See Providing alternative images if Adobe Flash isn’t available for more alternatives.)

But it's not possible to have a bandwidth control with HTML alone.

You must use javascript. Here is a plugin detection tool on JS.

http://www.oreillynet.com/pub/a/javascript/2001/07/20/plugin_detection.html

 
var isFlashInstalled = detectFlash();
if (isFlashInstalled)
{
  window.location = "main_with_flash.htm";
}
else
{
  window.location = "main_no_flash.htm";
}