Web browsers originally were designed to display only text and HTML
data. Later, they gained the ability to display certain kinds of
graphics. Then, when surfers began demanding fancier multimedia content
in Web pages, browsers began accommodating more kinds of information.
These new kinds of information were, and continue to be, defined by
MIME (Multipurpose Internet Multimedia Extension)-type designations.
Browsers handle some MIME-types natively while others get handed off to
special browser-expansion modules called plug-ins that surfers can
install selectively. Such selectivity poses a problem for Web
publishers. How do you play a RealAudio file for a visitor to your site
if that person doesn't have the RealAudio plug-in installed? What if a
surfer doesn't have the RealAudio plug-in, but can handle data of the
audio/x-wav MIME-type? What about ActiveX Controls? Well, since
two technologies work and playing together nicely is practically zero.