Some embodiments provide a system that executes a web application. During operation, the system loads the web application in a web browser and loads a native code module associated with the web application into a secure runtime environment. Next, the system writes a set of rendering commands to a command buffer using the native code module and concurrently reads the rendering commands from the command buffer. Finally, the system renders an image for use by the web application by executing the rendering commands using a graphics-processing unit (GPU).