[css-shapes-1] Clarify that invalid path strings are invalid, per WG resolution. w3c/fxtf-drafts#392

Author: fantasai

css-shapes-1/Overview.bs

@@ -466,8 +466,10 @@ Supported Shapes</h3>
 				<li>
 					The <dfn><<string>></dfn>  represents an
 					<a href="https://www.w3.org/TR/SVG11/paths.html#PathData">SVG Path data string</a>.
-					The path data string must be conform
-					to the grammar and parsing rules of SVG 1.1.
+					A path data string that does not conform to the
+					to the grammar and parsing rules of SVG 1.1,
+					or that does conform but defines an empty path,
+					is [=invalid=] and causes the entire ''path()'' to be [=invalid=].
 					The initial position is defined
 					by the first “move to” argument
 					in the path string.
@@ -1129,6 +1131,7 @@ Since <a href="https://www.w3.org/TR/2014/CR-css-shapes-1-20140320/">March 20th
 		<li>Added clarifications to shape-margin examples</li>
 		<li>Added margin=0 case for margin-box shape</li>
 		<li>Changed rules about degenerate shapes to use shape edges</li>
+		<li>Clarified that invalid path strings make the ''path()'' invalid.</li>
 	</ul>
 
 <h3 class="no-num" id="20140211">