Fix incorrect em unit calculation in CssBoxProperties

Author: chenjt2001

Source/HtmlRenderer/Core/Dom/CssBoxProperties.cs

@@ -1432,8 +1432,8 @@ public double ActualBorderSpacingVertical
         /// <returns></returns>
         public double GetEmHeight()
         {
-            return ActualFont.Height;
-        }
+            return ActualFont.Size * 96d / 72d;
+		}
 
         /// <summary>
         /// Ensures that the specified length is converted to pixels if necessary