https://drafts.csswg.org/css-transforms/#funcdef-rotate3d, rotate3d is defined as:

specifies a 3D rotation by the angle specified in last parameter about the [x,y,z] direction vector described by the first three parameters. A direction vector that cannot be normalized, such as [0,0,0], will cause the rotation to not be applied. 

The second sentence implies normalization, but the first could be more explicit I think.


(Down in https://www.w3.org/TR/css-transforms-1/#interpolation-of-transform-functions, it does talk about normalization for rotate3d:

"The transform functions matrix(), matrix3d() and perspective() get converted into 4x4 matrices first and interpolated as defined in section Interpolation of Matrices afterwards. 

For interpolations with the primitive rotate3d(), the direction vectors of the transform functions get normalized first. If the normalized vectors are equal, the rotation angle gets interpolated numerically.")