The engine that powers Firefox delivers accurate display of complex Web sites and provides support for new Web standards.
These pages conform to Strict XHTML 1.0. This standard is not supported by older browsers, and the display of these pages may be incorrect on old browsers. Images are in the PNG format, which may also be unsupported by some very old browsers (click here to see how this page should be rendered by a suitable browser).
The following browsers correctly handle these formats :
Why use these formats ? XHTML 1.0 is the first HTML specification based on XML. This standard is more strict and better specified that its predecessors, which ensure compability between browsers (the "flexibility" of previous formats made various browsers interpret them often differently). Since XHTML is now correctly supported by mainstream browsers, it is necessary to adopt it.
The PNG format replaced the GIF format since the owners of a related patent decided to claim royalties for its usage. PNG is an open format (no fear to see its usage limited by a commercial organisation), and offers more interesting technical features.
Never forget that the Internet and even computers wouldn't exist without strong and open standards. How could one expect two computers to interoperate without a shared language ? And how could one expect interoperable hardware and sofware if the specifications of these standards where not available ? Also, the fact that major standards remain open (i.e. available to everybody free of charge) is also the unique way to ensure that users will never be locked by a commercial company (which is sadly the case with the most widely used office format).