Bitstream Foundry runs on Microsoft's Office 365 platform; more specifically, it runs on an Office 365 E3 tenant. This site is built using the public site that is available to most Office 365 tenants. A fair amount of time and effort have been invested in branding and functionality, but the site you're browsing is entirely SharePoint 2013 under-the-hood.
Applying Responsive Design concepts is an important part of building a web site that looks good in a variety of browsers and on the multitude of web-capable devices that are available today. This site employs Kyle Schaeffer's SP Blueprint master page starter solution to deliver a responsive experience. Kyle's solution works particularly well for SharePoint 2013 public sites - especially those resident in Office 365.
Additional responsiveness is made possible through the use of some supplemental (CSS3) media queries. These media queries do things like alter the site's logo and hide images as screens get smaller - all in an attempt to improve the "mobile experience."
Although a variety of different script libraries and other downloadable files are needed to drive this site's functionality, an attempt has been made to remain "bandwidth conscious" where possible. Since the 2010 release, SharePoint has possessed the ability to retrieve scripts on-demand (SOD) as they are needed by a client browser. This site uses SharePoint's SOD capabilities in various places to retrieve scripts only as they are needed; this is done in an attempt to reduce both end-user waiting and bandwidth consumption.