Update align-content.md

docs/css/align-content.md

@@ -9,8 +9,10 @@
 
 <!-- CSSJSON.align-content.defaultValue -->
 
+按W3C规范，normal值在弹性布局（display: flex）中行为与stretch一致，详情参考：[CSS Box Alignment](https://drafts.csswg.org/css-align/#distribution-flex)。
+
 <!-- CSSJSON.align-content.unixTags -->
 
 <!-- CSSJSON.align-content.compatibility -->
 
-<!-- CSSJSON.align-content.reference -->
\ No newline at end of file
+<!-- CSSJSON.align-content.reference -->