[css-syntax] when to insert "/**/"

The Serialization section of css-syntax says:

   If the implementation does not preserve comments, it must insert the
   text "/**/" between the serialization of adjacent tokens when the two
   tokens are of the following pairs: ...

An implementation can preserve comments yet still need to insert "/**/" 
in places, for example with:

   p { var-a:/*a*/1; var-b:var(a)2; }

where the serialization of the computed value of 'var-a' on a <p> would 
be "/*a*/1/**/2".