“It is possible to use a GL context without a default framebuffer, in which case a framebuffer object must be used to perform all rendering. This is useful for applications needing to perform offscreen rendering.”
Can anyone please explain how to implement this?
That’s merely an extension which allows the default framebuffer in OpenGL ES 2.0 to be undefined. How you actually create a context with an undefined default framebuffer is up to your specific context creation system. The extension simply explains the behavior of such a context.