No server side. Only JS: a full-fledged computer emulator with Linux on board.
bellard.org/jslinux(attention, only chrome and FF4)
I played with it for a long time - it is not imitation, Linux behaves like a real Linux - compiled programs work, errors in them cause segmentation fault, damage to the root file system causes a storm of indignation in dmesg, etc.

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dd shows more than decent performance when working with memory - more than 40 Mb / s (don't forget that this is an emulator, and that this is JS in your browser!).
I never thought we would live to see the like.
... And now the orgy begins:
* Using local storage, we can organize disks (each key-value corresponds to one sector).
* Using a web-socket, we can create a para-virtualized network driver with access to the iron router and get a normal network.
* With the use of existing technologies (NUMA, DRBD, corosync) you can organize a computing cluster of browsers.
(By the way, it is not necessary to show the console in this case - you start the virtual machine on the client in the background, the virtual machine joins the cluster, starts counting, after it crashes - closing the browser - the cluster is automatically reconfigured).
I write this as Hochma, but in reality, if you have Linux and you have it, you have all the possibilities for anything. Any network applications, any systems of synchronization and management.
No one bothers to create a paravirtualized video driver using canvas, you have webGL for which you can write your own cuda version and get a pretty powerful acceleration of computations ...
Well, the final is the launch of chrome inside the emulated virtual machine.
So, the emulation reached the browsers ...