[css-overflow-3] Add (-webkit-)line-clamp

Part of #390

css-overflow-3/Overview.bs

@@ -684,6 +684,39 @@ Indicating Block-Axis Overflow: the 'block-overflow' property</h2>
 	to use as either an inline-level or block-level indicator
 	(in which case, it can capture events).
 
+
+<h2 id=line-clamp>
+Setting 'max-lines' and 'block-overflow' together: the 'line-clamp' property</h2>
+
+	<pre class=propdef>
+		Name: line-clamp
+		Value: none | <<integer>>
+		Initial: none
+		Applies to: see individual properties
+		Inherited: see individual properties
+		Animatable: see individual properties
+		Percentages: N/A
+		Media: visual
+		Computed value: see individual properties
+	</pre>
+
+	The 'line-clamp' property is a <a>shorthand</a>
+	for the 'max-lines' and 'block-overflow' properties.
+
+	<dl dfn-for=line-clamp dfn-type=value>
+		<dt><dfn>none</dfn>
+		<dd>Sets 'max-lines' to ''max-lines/none''
+			and 'block-overflow' to ''block-overflow/clip''.
+
+		<dt><dfn><<integer>></dfn>
+		<dd>Sets 'max-lines' to the specified <<integer>>
+			and 'block-overflow' to ''block-overflow/ellipsis''.
+
+	</dl>
+
+	For compatibility with legacy content, UAs that support 'line-clamp' must also support <dfn dfn-type=property>-webkit-line-clamp</dfn> as an alias.
+
+
 Privacy and Security Considerations {#priv-sec}
 ===============================================