diff --git a/src/document/css/parser.c b/src/document/css/parser.c
index 86e6b7f3..9e301e0c 100644
--- a/src/document/css/parser.c
+++ b/src/document/css/parser.c
@@ -170,7 +170,20 @@ struct selector_pkg {
 	struct css_selector *selector;
 };
 
-struct css_selector *
+/** Move a CSS selector and its leaves into a new list.  If a similar
+ * selector already exists in the list, merge them.
+ *
+ * \param sels
+ *   The list to which \a selector should be moved.  Must not be NULL.
+ * \param selector
+ *   The selector that should be moved.  Must not be NULL.  If it is
+ *   already in some list, this function removes it from there.
+ * \param watch
+ *   This function updates \a *watch if it merges that selector into
+ *   another one.  \a watch must not be NULL but \a *watch may be.
+ *
+ * \return \a selector or the one into which it was merged.  */
+static struct css_selector *
 reparent_selector(struct list_head *sels, struct css_selector *selector,
                   struct css_selector **watch)
 {