Convert fixed width grid to fluid
Add responsive toggled navigation menu
Rework hero and used-by sections for small screens
Break modules stylesheet into separate files
Create breakpoint-based responsive grid system

Preprocessors (Stylus and Sass included) don't support @extend in media
queries, so you can't silently include grid units for different content
areas based on screen size. Instead, we define separate grids for major
breakpoints, then @extend those classes within content areas. For
reference, see below:

http://csswizardry.com/2013/02/responsive-grid-systems-a-solution/
Add toggling menu system for small screens
Add responsive layout to getting started page
Add basic responsive styling for API documentation
Fix inline code sample backgrounds on API reference

The code samples were displaying much darker than was intended. I
suspect that Stylus changed their 'lighten' algorithm in a way that
necessitated the change, but I have no proof. Perhaps Stylus left a clue
in an alley or a doorway?
Remove background color from features menu on large screens
Add responsive tables to API Reference

This solution isn't particularly robust, but should work for the simple
two column tables in the API. At small screen sizes, each row becomes
a separate box, with table cells distinguished by color that matches
with the table headings at the top.
Style API sidebar for small screens
Replace icon images with FontAwesome
Base encode .woff and .ttf versions of icon font

Inlining fonts saves a request at a marginal increase to file size.
Because the icon font is only four characters, the size increase is
minor, and worth it! You can't encode .eot fonts though, which is why
it's left out.
Add new Modernizr build with touch and font-face detects

This build detects touch devices and font-face support, which will help
with fallback styles for mobile devices especially.
Prevent API sidebar from becoming fixed on screens that are too short
Remove Modernizr test for touch screens

I ended up not using the Modernizr touch screen test for mobile styling
on things like fixed position elements, because touch screen desktop
devices are a thing now. Thanks Windows 8.
Fix Stylus syntax error (embarrassing!)
Add icon font fallbacks

This handles browsers that don't support @font-face. Decorative icons
are hidden and substantive icons (like menu buttons) display text
instead.
Prevent screencast video backdrop from displaying over other content

During the screencast widget opening and closing animation, the backdrop
would leak out over the used by section and main home page content. This
is fixed by adding z-index and background color to those sections to
ensure that they're always a level above the video container.