Getting CAD models visualized on the Web usually requires some work, as CAD representations, such as BReps, differ from visualization formats, such as X3DOM’s BinaryGeometry or the X3D IndexedFaceSet.
To solve problems like this, you can use PythonOCC, a python package that comes with powerful CAD functionality. This includes, for instance, exporting STEP to X3D with just a few lines of code.
For a quick look, check out some simple examples here!
This also includes an improved support for exporting scenes to X3DOM.