[css-overflow-3] Define the legacy compat behavior of line-clamp

Based on Mozilla’s research, this should be sufficient to make
-webkit-line-clamp and line-clamp web compatible.

See w3c#2847

Author: frivoal

css-overflow-3/Overview.bs

@@ -992,11 +992,26 @@ Limiting Visible Lines: the 'line-clamp' shorthand property</h3>
 		</pre>
 	</div>
 
-	For compatibility with legacy content, UAs that support 'line-clamp' must also support <dfn dfn-type=property>-webkit-line-clamp</dfn> as an alias.
-
-	Issue(2847): It is not obvious that a naive -webkit- alias can work, due to complicated compatibility constraints.
-	Something more complicated might be needed.
-
+<h4 id=webkit-line-clamp>
+Legacy compatibility</h4>
+
+	For compatibility with legacy content,
+	UAs that support 'line-clamp' must also support the <dfn dfn-type=property>-webkit-line-clamp</dfn> property.
+	Like 'line-clamp', it is a shorthand of 'max-lines', 'continue', and 'block-ellipsis',
+	except that it sets 'continue' to ''-webkit-discard'' instead of ''discard''.
+
+	Additionally, for children (including anonymous children)
+	of boxes whose 'display' property computes to ''-webkit-box'' or ''-webkit-inline-box'',
+	the used values of the 'max-lines', 'continue', and 'block-ellipsis' properties
+	are taken from the computed values of the parent box;
+	the computed values of these properties on the box itself are ignored.
+
+	The <dfn value for=continue>-webkit-discard</dfn> value behaves identically to ''discard'',
+	except that it only takes effect
+	if the computed value of the 'display' property on the parent
+	is ''-webkit-box'' or ''-webkit-inline-box''
+	and the computed value of the '-webkit-box-orient' property on the parent
+	is ''-webkit-box-orient/vertical''.
 
 <h3 id="max-lines">
 Forcing a Break After a Set Number of Lines: the 'max-lines' property</h3>