[cssom] Cleanup some serialization bits.

Do not hardcode hz as the canonical unit for <frequency>, nor mm as the
canonical unit for <length>. That doesn't reflect reality in any way.

Sprinkle some issues about preserving units in specified values, which UAs do
and probably CSSOM should spec, since there's no way to serialize relative stuff
as px when values are not computed, plus UAs don't preserve units after
computation.

Fixes w3c#1554

Author: emilio

cssom-1/Overview.bs

@@ -2517,7 +2517,10 @@ depends on the component, as follows:
  <a lt="ASCII lowercase">converted to ASCII lowercase</a>.
 
  <dt>&lt;angle>
- <dd>The &lt;number> component serialized as per &lt;number> followed by the unit in canonical form as defined in its respective specification.
+ <dd>
+  The &lt;number> component serialized as per &lt;number> followed by the unit in canonical form as defined in its respective specification.
+
+  Issue: Probably should distinguish between specified and computed / resolved values.
 
  <dt>&lt;color>
  <dd>
@@ -2599,8 +2602,10 @@ depends on the component, as follows:
 
 
  <dt>&lt;frequency>
- <dd>The frequency in hertz serialized as per &lt;number> followed by
- the literal string "<code>hz</code>".
+ <dd>
+  The &lt;number> component serialized as per &lt;number> followed by the unit in its canonical form as defined in its respective specification.
+
+  Issue: Probably should distinguish between specified and computed / resolved values.
 
  <dt>&lt;identifier>
  <dd>The identifier
@@ -2613,19 +2618,9 @@ depends on the component, as follows:
 
  <dt>&lt;length>
  <dd>
-  A length of zero is represented by the literal string
-  "<code>0px</code>".
-
-  Absolute lengths: the number of millimeters serialized as per
-  &lt;number> followed by the literal string "<code>mm</code>".
-
-  Issue: Rumor has it absolute lengths will become relative
-  lengths. Centimeters would be compatible with &lt;resolution>...
-
-  Relative lengths: the &lt;number> component serialized as per
-  &lt;number> followed by the unit in its canonical form as defined in its
-  respective specification.
+  The &lt;number> component serialized as per &lt;number> followed by the unit in its canonical form as defined in its respective specification.
 
+  Issue: Probably should distinguish between specified and computed / resolved values.
 
  <dt>&lt;number>
  <dd>