This demo does not do any drawing and is not interactive. Its purpose is to show the implementation-dependent values associated with certain constants on the device on which it is run. The values are obtained by calling gl.getParameeter(). Scroll down the list of constants to see all available values.
The value for the first constant, RENDERER, is the name of the system that does WebGL rendering in the web browser that you are using to view this demo.
The remaining constants represent limits that are enforced by the WebGL renderer. Many of them are limitations on GLSL shaders. For example, MAX_VERTEX_ATTRIBS gives the maximum number of attributes that can be used in a vertex shader. The limitations on texture units refer to the number of values of sampler variables in the shader.
(The word "ALIASED" in ALIASED_POINT_SIZE_RANGE and ALIASED_LINE_WIDTH_RANGE is a leftover from OpenGL, where the limits applied to antialised points and lines. In WebGL, the limits have nothing to do with aliasing. They apply to all points and lines.)
The demo uses a WebGL 1.0 drawing context. It is possible that some values might differ for a WebGL 2.0 context on the same device. (On my computer, that's true for ALIASED_LINE_WIDTH_RANGE.)