1. Is it against adsense rules to make the 728x90 ad responsive? 2. if not, how do I do it? 3. Is there anything bad about making the 728x90 ad responsive?
Why not just use their own responsive ads? They have both responsive ad and link units. Just choose Responsive from the drop down.
728x90 Its Leader board.It is one of the best ad units.If you display at top of the site below site navigation and above content with some space left in between both of them, so that users don't get confused.Blend the ads very well until you see good click through rates.