[Alden Kroll] Two-dimensional HUD elements present a particular art problem, because they have to look good and sharp no matter what resolution the user is running their game at. Given today's availability of high resolution wide-screen displays, this can require a lot of texture memory and a lot of work anticipating different display resolutions. The problem for Team Fortress 2 was even more daunting because of our desire to include a lot of smooth curved elements in our HUD. We developed a new shader system for drawing 'line art' images. The system allows us to create images at a fixed resolution that produced smooth silhouettes even when scaled up to a very high resolution. This shader system also handles outlining and drop-shadows, and can be applied in the 3D space to world elements such as signs.
As far as I know the signs in tf2 don't use the shader system they built for the hud, but they could.