Updated CSS Selector help (markdown)

CSS-Selector-help.md

@@ -16,6 +16,17 @@ So for example if the HTML is
 
 the selector would be `.points #interesting`
 
+In this example
+
+```html
+<span class="chapter lastchapter">
+  Chapter 53
+</span>
+```
+
+these three selectors will match : `.chapter`, `.lastchapter`, `.chapter.lastchapter`. This last one will only match when both CSS classes match.
+`.chapter lastchapter` is wrong in this case.
+
 # Select/extract just the HREF from `<A>` tag using xpath
 
 if you have