Absolutely positioned elements with 'left' and 'right defined, but no width, now correctly resize the element, as per section 10.3.7 of the CSS 2.1 spec.

Source/Core/LayoutEngine.cpp

@@ -465,6 +465,7 @@ void LayoutEngine::BuildBoxWidth(Box& box, Element* element, float containing_bl
 	// Determine if the element has an automatic width, and if not calculate it.
 	bool width_auto;
 	int display_property = element->GetProperty< int >(DISPLAY);
+
 	if (display_property == DISPLAY_INLINE_BLOCK)
 	{
 		width_auto = false;
@@ -529,6 +530,30 @@ void LayoutEngine::BuildBoxWidth(Box& box, Element* element, float containing_bl
 			box.SetEdge(Box::MARGIN, Box::RIGHT, margin);
 	}
 
+	// In the case we're an absolutely positioned element, we need to check our width.
+	int position_property = element->GetProperty< int >(POSITION);
+	if (display_property == DISPLAY_BLOCK && (position_property == POSITION_ABSOLUTE || position_property == POSITION_FIXED))
+	{
+		if (width_auto)
+		{
+			const Property* leftProp = element->GetLocalProperty(LEFT);
+			const Property* rightProp = element->GetLocalProperty(RIGHT);
+
+			// Check for left and right properties both being defined.
+			if ((leftProp != NULL) && (rightProp != NULL))
+			{
+				content_area.x = containing_block_width - (box.GetCumulativeEdge(Box::CONTENT, Box::LEFT)
+					   + box.GetCumulativeEdge(Box::CONTENT, Box::RIGHT)
+					   + leftProp->Get<float>() + rightProp->Get<float>());
+				content_area.x = Math::Max(0.0f, content_area.x);
+			}
+		}
+		else
+		{
+			// For now, we're ignoring the over-constrained situation
+		}
+	}
+
 	// Clamp the calculated width; if the width is changed by the clamp, then the margins need to be recalculated if
 	// they were set to auto.
 	float clamped_width = ClampWidth(content_area.x, element, containing_block_width);