[css-variables] Add issue about the global keywords in custom properties.

Author: tabatkins

css-variables/Overview.html

@@ -155,6 +155,15 @@ <h2 id="introduction" data-level="1"><span class="secno">1 </span>
 		such that they just never animate,
 		not even with the "just flip at 50% progress" like everything else?
 
+	</p><p class="issue">
+		Should the global values initial/inherit/default be interpreted?
+		I'm guessing no,
+		because we're not meant to interpret *anything*.
+		You can get switch it back to inheriting by setting it to a guaranteed-invalid value,
+		like <span data-autolink="maybe" class="css">var-foo: var(foo);</span>,
+		but you can't get <span data-autolink="maybe" class="css">default</span> behavior.
+
+
 </p><p>	A <dfn id="custom-property">custom property</dfn> is any property
 	whose name starts with "var-",
 	and contains at least one additional character.

css-variables/Overview.src.html

@@ -84,6 +84,15 @@ <h2 id='defining-variables'>
 		such that they just never animate,
 		not even with the "just flip at 50% progress" like everything else?
 
+	<p class='issue'>
+		Should the global values initial/inherit/default be interpreted?
+		I'm guessing no,
+		because we're not meant to interpret *anything*.
+		You can get switch it back to inheriting by setting it to a guaranteed-invalid value,
+		like ''var-foo: var(foo);'',
+		but you can't get ''default'' behavior.
+
+
 	A <dfn>custom property</dfn> is any property
 	whose name starts with "var-",
 	and contains at least one additional character.