How one should prepare the animations for export to xml3d? Does the exporter supports pose library? Or the exported animations are keyframe based. Is there any documentation where I can check how exporter maps the animation data?
I guess you are referring to this exporter: https://github.com/ksons/xml3d-blender-exporter/
This exporter supports armatures in many but probably not all ways supported by Blender. Here is a very coarse mapping from blender objects to XML3D elements: https://github.com/ksons/xml3d-blender-exporter/wiki/Mappings