[selectors] Fix definition of :focus

Closes w3c#1240

Author: frivoal

selectors/Overview.bs

@@ -2035,12 +2035,15 @@ The Input Focus Pseudo-class: '':focus''</h3>
 	(accepts keyboard or mouse events, or other forms of input).
 
 	There may be document language or implementation specific limits on
-	which elements can acquire '':focus''.
-	For example, [[HTML5]] defines a <a href="http://www.whatwg.org/html/selectors.html#selector-active">list of activatable elements</a>.
-
-	If the document language has defined additional ways by which an element can match '':active'',
-	the same ways must apply to elements matching '':focus'' as well,
-	except that the parent of an element that matches '':focus'' must not match '':focus''.
+	which elements can acquire :focus.
+	For example, [[HTML]] defines a list of <a href="https://html.spec.whatwg.org/multipage/interaction.html#focusable-area">focusable areas</a>.
+
+	Document languages may define additional ways in which an element can match '':focus'',
+	except that the '':focus'' pseudo class must not automatically propagate to the parent element
+	(see '':focus-within'' if matching on the parent is desired).
+	Note: it may still apply to the parent element
+	if made to propagate due to other mechanisms,
+	but not merely due to being the parent.
 
 <h3 id="the-focusring-pseudo">
 The Input Focus-Ring Pseudo-class: '':focus-ring''</h3>