To make it appear awesome in every browser there is, whether it's a laptop, desktop, phone, and tablet. If it's mobile, it needs to respond to the width of the screen.
Leave it alone. I wrap your HTML into Bootstrap's framework, clear out your old CSS, then create a custom.css file to modify your old layout to be Responsive.
The raven configures the layout of your existing website, and determines what Bootstrap framework is needed to make it Responsive and adhere to your existing layout.
The rat clears out your old CSS file, creates a custom.css so that your existing HTML remains the same. Developers don't have to relearn semantic markup. Yay!