XRender is always on, and used automatically through Cairo.
WebGL is the feature that gets turned on/off depending on your driver.
Free Mesa-based drivers are whitelisted if the Mesa version is at least 7.10 and the driver is not Gallium-based. See the discussion in bug 645407. Regarding Gallium, see bug 624935.
For example, the free Intel driver that ships in Ubuntu 11.04 is whitelisted.
For the NVIDIA driver, versions 257.21 and newer are whitelisted, exactly like on Windows (see above).
For the FGLRX (proprietary ATI) driver, we whitelist the newer versions that implement OpenGL 3.0 or newer. Indeed, FGLRX does not return any version number of its own, so we had to use the OpenGL version as a differentiator.