Added official w3c spec reference to question: attributes in HTML questions (yangshun#74)

Author: alexprut

front-end/interview-questions.md

@@ -99,6 +99,7 @@ These days, using `data-` attributes is not encouraged. One reason is that users
 ###### References
 
 - http://html5doctor.com/html5-custom-data-attributes/
+- https://www.w3.org/TR/html5/dom.html#embedding-custom-non-visible-data-with-the-data-*-attributes
 
 ### Consider HTML5 as an open web platform. What are the building blocks of HTML5?