The technology for building optimal viewing experience websites is now widely known as Responsive Design. However there are more than one way to accomplish excellent interface without jeopardizing the concept you have in mind.
Most popular CSS frameworks are Bootstrap, Foundation and Skeleton, but even they have limitations. It takes experience with each of these frameworks to know when and how to implement them. In fact, including a framework inside the code of an old website could mean breaking the entire interface, even worse before implementing it.
Also a lot of new websites require designs that these frameworks just don’t offer, so you would need some mostly jQuery scripts like Modernizr or FitVids for optimal video experience. You also may need the Columnizer script for columns inside posts that are not pre-formated columns.
The road toward web viewing perfection doesn’t stop here though. Newer scripts appear, that add animated parts that are also responsive. The more the web enhances the more web developers are in demand for technologies that can turn an HTML site into a Flashy animated application viewable on all screens and size.