[css-properties-values] Simplify the Conditional Rules section a bit.

Author: tabatkins

css-properties-values-api/Overview.bs

@@ -338,24 +338,24 @@ which can start or interrupt a CSS transition.
 Conditional Rules {#conditional-rules}
 --------------------------------------
 
-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: "<color>"`,
-a rule like `@supports (--foo: 1em) {...}`
-will still evaluate as true and apply those styles.
-
-Note: This matches the [[#parsing-custom-properties|parsing behavior]] of registered custom properties.
-A registered custom property specified as `"<color>"`
-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 parsing behavior,
-it thus also accepts all values as valid regardless of the registered syntax.
+As stated in [[#parsing-custom-properties]],
+both unregistered and [=registered=] [=custom properties=]
+accept (almost) all possible values at parse-time.
+[=Registered=] [=custom properties=] only apply their syntax at [=computed value=] time.
+
+So, all [=custom properties=],
+regardless of whether they're [=registered=] or unregistered,
+will test as "true" in an ''@supports'' rule,
+so long as you don't violate the (very liberal) generic syntax for [=custom properties=].
+
+<div class=example>
+	For example,
+	even if a custom property is registered
+	with <code highlight=css>syntax: "&lt;color>";</code>,
+	a rule like `@supports (--foo: 1em) {...}`
+	will still evaluate as true and apply those styles,
+	because the declaration <em>does</em> successfully parse as a valid property.
+</div>
 
 
 Substitution via ''var()'' {#substitution}