Wikipedia:
Responsive web design (RWD) is an approach to web design aimed at allowing desktop webpages to be viewed in response to the size of the screen or web browser one is viewing with. In addition it's important to understand that Responsive Web Design tasks include offering the same support to a variety of devices for a single website. As mentioned by the Nielsen Norman Group: content, design and performance are necessary across all devices to ensure usability and satisfaction.
PSM has a consistently high number of page views every day, usually around 900-1000.
The breakdown of different screen resolutions for the last 500 page views (the limit of the system logs) is:
From this you can see that tablet and smart phone users account for just over 50% of the page views. These are screen sizes with a width of 768pixels or less.
It is essential that pages on PSM respond to these visitors screens for optimal viewing.