[css-backgrounds-4] Added background-tbd property as a shorthand for background layers (w3c#9084)

The name of the property is still under discussion, so to avoid implementations, it got named `background-tbd` for now.

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -185,6 +185,62 @@ Painting Area: the 'background-clip' property</h3>
 		</dd>
 	</dl>
 
+<h3 id='background-layers'>
+Background Image Layers: the 'background-tbd' shorthand property</h3>
+
+	<pre class="propdef">
+		Name: background-tbd
+		Value: <<bg-layer>>#
+		Initial: see individual properties
+		Applies to: all elements
+		Inherited: no
+		Percentages: see individual properties
+		Computed value: see individual properties
+		Animation type: see individual properties
+	</pre>
+
+	<p>The 'background-tbd' property is a [=shorthand property=]
+	that sets all the same properties as the 'background' shorthand
+	except for 'background-color',
+	allowing authors to easily declare and position background images
+	while letting 'background-color' cascade through independently.
+
+	Issue:
+		The name of this property is discussed in
+		<a href="https://github.com/w3c/csswg-drafts/issues/9083">issue 9083</a>.
+
+	<div class="example" id="background-layer-example">
+		This example sets two background layers later in the cascade.
+		By using 'background-tbd', the previously set 'background-color'
+		won't be overridden.
+
+		<pre class="lang-css">
+		p {
+			background-color: green;
+		}
+
+		p {
+			background-tbd:
+				url(a.png) top left,
+				url(b.png) top left no-repeat;
+		}
+		</pre>
+	</div>
+
+	<div class="invalid example" id="invalid-background-layer-example">
+		This example tries to set the background color in addition to
+		the background image. But for that to work,
+		'background' needs to be used instead of 'background-tbd'.
+		So the 'background-tbd' declaration will be dropped.
+
+		<pre class="lang-css">
+		p {
+			background: url(pass.png) green;   /* valid */
+			background-tbd: url(fail.png) red; /* invalid */
+		}
+		</pre>
+	</div>
+
 Issue: Should a <a href="https://lists.w3.org/Archives/Public/www-style/2011Sep/0331.html">'background-repeat: extend'</a> be added?
 
 <h2 id="changes">
@@ -196,6 +252,7 @@ Additions since [[CSS3BG]]</h3>
 	* turned 'background-position' into a shorthand and added physical and logical longhands
 	* added logical keywords to <<bg-position>>
 	* added 'background-clip'
+	* added 'background-tbd'
 
 <h2 id="acknowledgments">Acknowledgments</h2>