If I have an element with width: 1cm, then does its computed value reify to CSS.cm(1) (which serializes to pixels)? In CSSOM, we only need to deal with canonical units because serialization uses canonical units, but I think this doesn't apply to reified objects in Typed OM?

If this is the required behaviour, then AFAIK this is difficult to implement in Blink because we don't retain unit information in computed style. I don't know about the other browsers.