For the widest range of existing solutions, you will not beat the conversion of your models into VRML .
There are plugins in this format that browsers support in IE 3 and Netscape 4. There are a large number of free and commercial plugins that handle this format. If you do not want the user to rely on plugins, there are also java applets based on applets.
There should be plenty of tools that also convert your models to VRML, even if you need to save them in an intermediate format first.
The main disadvantage of VRML is the lack of fantastic new features such as shaders, bones and complex texturing, but whether these things are needed depends on what you are trying to achieve.
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