Why do you believe it is impossible to display a web page of text on screen without a 3D graphics accelerator adapter card? Haven't you ever used a computer without a Nvidia in it before? A young kid like you might be surprised to learn that there was a time not so long ago when most computers didn't have 3D accelerator cards, and that they were still more than capable of rendering a page of information and driving their displays.
Sure it is possible to display the page with text, it is matter of what compatibility level one wants to achieve in regards to modern Web platform in pages that might have 3D and media content into them, which includes some forms of CSS styles as well.
Ultimately everything can be done via software rendering as well, depending on how much one is willing to wait.
OpenGL already has a client/server model (i remember a friend of mine once showing me running some OpenGL stuff from his SGI to his Linux machine... or the other way around, it has been years :-P) and i can't think of anything in WebGL that couldn't work in the same way (though i only used WebGL very little). Sending commands and data over a file handle fits this fine.
You aren't going to be sending vertices through the pipe every frame.
You are going to be sending commands to create the vertex buffers and upload textures (with the relevant data) and then most of your commands will be referencing those. Which is basically exactly what discrete GPUs already do anyway.
IIRC, unlike OpenGL, WebGL doesn't even allow you to do things in another way.
What X Windows did isn't very relevant because the use is very different. It'd only be somewhat comparable if the WebGL programs would treat WebGL as a way to stream new frame data for every frame (e.g. video). Also all X Windows communication with clients is done via commands sent through unix sockets which have similar performance to pipes.
