diff --git a/src/hb-ot-layout.h b/src/hb-ot-layout.h
index 8999eaf69..f8fab24bc 100644
--- a/src/hb-ot-layout.h
+++ b/src/hb-ot-layout.h
@@ -278,7 +278,7 @@ Xhb_ot_layout_lookup_position (hb_font_t            *font,
 #endif
 
 /* Optical 'size' feature info.  Returns true if found.
- * XXX Add URL. */
+ * http://www.microsoft.com/typography/otspec/features_pt.htm#size */
 hb_bool_t
 hb_ot_layout_get_size_params (hb_face_t *face,
 			      uint16_t  *data /* OUT, 5 items */);