[css-inline-3] Define synthesize box for atomic inline boxes. w3c#3611

Author: fantasai

css-inline-3/Overview.bs

@@ -352,6 +352,11 @@ Alignment Baseline Type: the 'alignment-baseline' longhand</h4>
 			Match the top of the box to the top of the parent's content area.
 	</dl>
 
+	For the purposes of transverse alignment within a line box,
+	an [=atomic inline=] that has no [=baseline sets=]
+	uses baselines [=synthesize baseline|synthesized=]
+	from its [=margin box=].
+
 	For the following definitions, the <a>alignment subtree</a>
 	is as defined in [[!CSS2]].