[web-animations-1] Clarify that unsupported pseudo-elements are also invalid

See discussion in #4745.

Author: birtles

web-animations-1/Overview.bs

@@ -4692,9 +4692,9 @@ interface KeyframeEffect : AnimationEffect {
                 object? keyframes,
                 optional (unrestricted double or KeyframeEffectOptions) options = {});
     constructor(KeyframeEffect source);
-    attribute Element? target;
-    attribute CSSOMString? pseudoElement;
-    attribute CompositeOperation             composite;
+    attribute Element?           target;
+    attribute CSSOMString?       pseudoElement;
+    attribute CompositeOperation composite;
     sequence<object> getKeyframes();
     void             setKeyframes(object? keyframes);
 };
@@ -4844,11 +4844,18 @@ interface KeyframeEffect : AnimationEffect {
     sets the [=target pseudo-selector=] of the [=animation effect=]
     to the provided value after applying the following exceptions:
 
-    *   If the provided value is not `null` or a syntactically valid
-        <<pseudo-element-selector>> the user agent must [=throw=]
-        a {{DOMException}} with error name {{SyntaxError}} and leave the
+    *   If the provided value is not `null`
+        and is an invalid <<pseudo-element-selector>>,
+        the user agent must [=throw=] a {{DOMException}}
+        with error name {{SyntaxError}} and leave the
         [=target pseudo-selector=] of this [=animation effect=] unchanged.
 
+        Note, that invalid in this context follows the definition of
+        an [=invalid selector=] defined in [[!SELECTORS-4]] such that
+        syntactically invalid pseudo-elements as well as pseudo-elements
+        for which the user agent has no usable level of support are
+        both deemed invalid.
+
     *   If one of the legacy Selectors Level 2 single-colon selectors
         (':before', ':after', ':first-letter', or ':first-line') is
         specified, the [=target pseudo-selector=] must be set to the