According to http://my.opera.com/core/blog/2010/03/03/everything-you-need-to-know-about-html5-video-and-audio-2

"Opera 10.50 has now been released on Windows, and it supports the HTML5 video and audio elements."

Did anyone get this to work under 98?

I thought HTML 5 was new in 10.6 and might have been the reason 10.6 did not work under 98. But if it did work in 10.5, it is not the 98 incompatibility with 10.6.

Here you've got a build with HTML5 audio and video that should work on vanilla 98: 

http://snapshot.opera.com/webm/Opera_1054_21868_classic-WebM.exe

If HTML5 vids don't play in Opera 10.54 Final, you can replace the GStreamer folder with the one that is in this build and it should all work correctly.