For some time some companies/developer groups didn't want to support "browser rendered/interpreted" stuff natively because of security.
On the other hand this gave ActiveX a great head start since people basically coded the needed stuff via ActiveX object.
As now they changed their mind including Opera ASA who is the original company behind that stance against that stuff, it takes time to catch up.
If you are interested I can find the original Opera persons (who invented the CSS) blog posting. I bet it is still there.
Also until Yahoo, Google, Amazon, MS Live, AOL (big 5) support this thing it was basically a "geek fantasy". There are people still laughing/ criticizing Thinkfree Office which does this thing and many stuff via JAVA (real one,not applet!) and AJAX for years. http://www.thinkfree.com
, as the basic functionality is free now, I don't hesitate to give link. It is at least a great MS Word Viewer ;) Also it is a great site to test Omniweb's Java support since everything is used to the limit. BTW I just checked, site is offline until 4 AM PDT (whatever it is), here another reason why this stuff wasn't taken serious until big sites adapted it. ;)
Just imagine Macromedia/Adobe SVG somehow becomes widely used in 4-5 weeks, e.g. 10.000% increase in SVG content. Guess what happens? Every OS X browser will fail since (Adobe) SVG is broken since Tiger released and nobody cares.