Added note as to picture element source order and that the img elemen…

…t must always be present. (#1379)

Fix #1263, #1264

Thank you

.gitignore

@@ -1 +1 @@
-single-page.html
+single-page.html

sections/semantics-embedded-content.include

@@ -462,6 +462,9 @@
   which image resource to use, based on the screen pixel density, <a>viewport</a> size, image
   format, and other factors. It <a>represents</a> its children.
 
+  The user agent first tries to match an image source contained within the <{source}> attribute and use that,
+  but if none is found, it falls back to what is contained within the <{img}> element, which must be present.
+
   <div class="example">
     The following example utilizes <a for="image">art direction</a> to provide the appropriate
     image at a particular <a>viewport</a> for small and large screens.