Web pages are not picture, those are not "art". Web sites provide "function" to users.
Especially, EC sites have a mission; let customer know and understand products and services, let them buy.
End-users go back and forth among "screens": PC, tablet, smartphone and TV.
Especially, for web designer/developer, 3 screens (PC, tablet and smartphone) are very important development target screen to manage.
Are you going to implement all functions of desktop site to tablet and smartphone?
No, it WON'T work. Do you know why?
Why you should avoid using "Responsive Design"? Are you familiar with mobile network limitation? (What "packet drop" makes your smartphone/tablet browsing?)
This slide explain what happened for e-commerce recently, and what kind of things web designer/developer should consider for building/revising web site. (written in Japanese)
Clipping is a handy way to collect important slides you want to go back to later.