[css-nesting-1] Add error-handling for inserting or setting selector on a nested style rule.

Author: tabatkins

css-nesting-1/Overview.bs

@@ -1133,24 +1133,27 @@ must return the result of
 invoking [=insert a CSS rule=] <var>rule</var>
 into the [=CSSRule/child CSS rules=] at <var>index</var>.
 
+
 The <dfn method for=CSSStyleRule>deleteRule(<var>index</var>)</dfn> method
 must [=remove a CSS rule=] from the [=CSSRule/child CSS rules=] at <var>index</var>.
 
 Note: Serialization of {{CSSStyleRule}} with nested rules
 are already well-defined by [[CSSOM]],
 via [=serialize a CSS rule=].
 
-<div class=issue>
-	Whether we attach .cssRules to CSSStyleRule,
-	as described here,
-	or to CSSStyleDeclaration
-	(to be consistent with future work
-	allowing `style` attributes to have nested rules)
-	is still an open question.
-
-	Also, attaching it to .style
-	means existing code setting <code>rule.style=""</code>
-	and expecting
-
-	(<a href="https://github.com/w3c/csswg-drafts/issues/7850">Issue 7850</a>)
-</div>
+Note: The restriction on what a [=nested style rule's=]
+selector can start with
+counts as a "constraint imposed by CSS"
+in step 5 of [=insert a CSS rule=]
+(when invoked by anything that takes [=nested style rules=],
+not just {{CSSStyleRule}} itself).
+
+When setting {{CSSStyleRule/selectorText}},
+if the {{CSSStyleRule}} is a [=nested style rule=],
+and the returned group of selectors
+starts with a selector
+that starts with an ident or function token,
+do nothing and return.
+
+Issue: The above paragraph will be inlined into the CSSOM algo,
+rather than monkey-patched.