When something not working as expected

Was helping an friend on some JavaScript code. was using yui-dom-event and same code was working differently on ie and ff. since I was helping remotely I couldn’t guess more out of it. Finally after 1 day of full calls and chats finally asked him to expose the url. finally figured some html tags were being closed improperly. so moral of the story is, “if your well known library behaving weirdly in different browsers first thing you should look at is, generated html source”.