Updated background-size to support two arguments.

css/tests.css

@@ -138,6 +138,7 @@ h2 {
   max-width: none;
 }
 #test-background-size {
-  background-size: 100px;
-  background-size: 6.25rem;
+  background-image: linear-gradient(90deg, transparent 50%, rgba(255,255,255,0.5) 50%);
+  background-size: 500px 500px;
+  background-size: 31.25rem 31.25rem;
 }

index.styl

@@ -125,7 +125,7 @@ font-size()
 
 /* Background size */
 background-size()
-	abstract-longhand('background-size', arguments)
+	abstract-shorthand('background-size', arguments)
 
 /* Max width */
 

tests.html

@@ -84,7 +84,7 @@
 	</section>
 	<section>
 		<h2>Background size</h2>
-		<div id='test-background-size' data-assert='background-size: 100px'>background-size 100px</div>
+		<div id='test-background-size' data-assert='background-size: 500px 500px'>background-size 500px 500px</div>
 	</section>
 	<script src='dom-style-test-runner.js'></script>
 

tests.styl

@@ -120,4 +120,6 @@ h2
 
 // background-size
 #test-background-size
-	background-size 100px
\ No newline at end of file
+	background-image: linear-gradient(90deg, transparent 50%, rgba(255,255,255,.5) 50%);
+	background-size: 500px 500px;
+	// background-size 100px
\ No newline at end of file