[css-properties-values-api] Specify how registered properties work in @supports. Fixes #118.

Author: tabatkins

css-properties-values-api/Overview.bs

@@ -401,6 +401,31 @@ matches to the specified value.
 For list values, the computed value is a list of the computed values of the
 primitives in the list.
 
+
+Registered Properties and @supports {#supports}
+-----------------------------------------------
+
+Unregistered custom properties are “always valid”
+when tested via ''@supports'';
+a rule like `@supports (--foo: red) {...}`
+is always true
+so long as you don't violate the (very liberal) syntax for custom properties.
+
+Registering a custom property does not change this.
+Even if a custom property is, for example,
+registered with `syntax: ""`,
+a rule like `@supports (--foo: 1em) {...}`
+will still evaluate as true and apply those styles.
+
+Note: This matches the parsing behavior of registered custom properties.
+A registered custom property specified as `""`
+but written as `--foo: 1em;` will still be accepted as valid at parse-time,
+but be automatically [=invalid at computed-value time=].
+As ''@supports'' tests parse behavior,
+it thus also accepts all values as valid regardless of the registered syntax.
+
+
+
 Behavior of Custom Properties {#behavior-of-custom-properties}
 ==============================================================