bvh

Motion capture files (BVH, Biovision Hierarchy) as clips. Decades of mocap are in this format (the CMU library, Bandai Namco's research set, most capture suites), and a BVH read here is the same thing as a clip read from a GLB (glb-clips): a GlbClipSource, its own skeleton and its clip. So it plays on any body through bakeClip or retargetClip, which read which bone is which from the names (CMU's and Mixamo's spellings both work).

• readBvh(text) reads one. Rotations are Euler angles in the order each joint's CHANNELS line lists them (BVH's rule), turned into quaternions. Units are guessed from the skeleton's height (a person stands 1 to 2.5 m; centimetres and inches are scaled to metres; anything else is scaled to a 1.8 m person), or set with scale.
• writeBvh(source, clip) writes any clip source out (a capture, a UAL clip) for Blender, MotionBuilder and other tools. Rest is BVH's: every joint unturned, offsets along the world axes, so a skeleton whose joints are turned at rest (UAL's, any glTF's) is written as its turns since rest.

Pure, no engine.