[css-variables] Custom properties should *not* serialize "exactly as specified"

Consider this code:

var s = document.body.style;
s.setProperty("--a", "a(");
s.setProperty("--b", "b");

The spec says "Specified values of custom properties must be serialized exactly as specified by the author". That's what Blink does, but it's bad!

s.getPropertyValue("--a"); // "a("
s.getPropertyValue("--b"); // "b"
s.cssText; // "--a:a(; --b:b;"

s.cssText = s.cssText;

s.getPropertyValue("--a"); // "a(; --b:b;"
s.getPropertyValue("--b"); // ""
s.cssText; // "--a:a(; --b:b;;"

s.cssText = s.cssText;

s.getPropertyValue("--a"); // "a(; --b:b;;"
s.getPropertyValue("--b"); // ""
s.cssText; // "--a:a(; --b:b;;;"

s.cssText = s.cssText;

s.getPropertyValue("--a"); // "a(; --b:b;;;"
s.getPropertyValue("--b"); // ""
s.cssText; // "--a:a(; --b:b;;;;"

I think Firefox is better, since it round-trips:

s.getPropertyValue("--a"); // "a()"
s.getPropertyValue("--b"); // "b"
s.cssText; // "--a: a(); --b: b;"

s.cssText = s.cssText;

s.getPropertyValue("--a"); // "a()"
s.getPropertyValue("--b"); // "b"
s.cssText; // "--a: a(); --b: b;" 

Start with the CSS Custom Properties section linked in the issue, then reproduce the shown setProperty and cssText round-trip cases. Compare the described Blink and Firefox behavior and determine the specification change needed so custom properties round-trip without consuming later declarations. Done means the expected serialization behavior is agreed and reflected in the draft.