[css-nesting] added a :not, :is, and && error sample

Author: Adam Argyle

css-nesting-1/Overview.bs

@@ -225,6 +225,20 @@ Direct Nesting {#direct}
 		   :is(.foo, .bar) + .baz,
 		   :is(.foo, .bar).qux { color: red; }
 		 */
+
+		ul, ol {
+			&:not(.foo,.bar) > .baz { color: red; }
+		}
+		/* equivalent to
+			 :is(ul,ol):not(.foo,.bar) > .baz { color: red; }
+		 */
+
+		.foo {
+			&:is(.bar, &.baz) { color: red; }
+		}
+		/* equivalent to
+			 .foo:is(.bar, .foo:is(.baz)) { color: red; }
+		 */
 		</pre>
 
 		But the following are invalid:
@@ -248,6 +262,12 @@ Direct Nesting {#direct}
 		}
 		/* Invalid because the second selector in the list doesn't
 		   contain a nesting selector. */
+
+		.foo {
+			color: red;
+			&&.bar { color: blue; }
+		}
+		/* Invalid because the 1st & isn't at the start of the selector */
 		</pre>
 	</div>