"""
translate3d(<translation-value>, <translation-value>, <translation-value>)
specifies a 3D translation by the vector [tx,ty,tz], with tx, ty and tz being the first, second and third translation-value parameters respectively. Note that <percentage> values are not allowed in the tz translation-value, and if present will cause the propery value to be invalid.

...

translateZ(<translation-value>)
specifies a translation by the given amount in the Z direction. Note that <percentage> values are not allowed in the translateZ translation-value, and if present will cause the propery value to be invalid.
"""
http://dev.w3.org/csswg/css3-3d-transforms/#transform-functions

It would be simpler if this just read

"""
translate3d(<translation-value>, <translation-value>, <length>)
specifies a 3D translation by the vector [tx,ty,tz], with tx, ty and tz being the first, second and third parameters respectively.

...

translateZ(<length>)
specifies a translation by the given amount in the Z direction.
"""

Currently it says <translation-value>, but then says in the prose that percentages are disallowed.  <translation-value> is just <length> or <percentage> anyway, so why not simplify and just say it has to be <length> with no extra prose?