A Linux system running over JavaScript

Why be worried? Is creating a virtual machine from a browser any more or less of a cause for concern than "normal" ways of starting virtual machines?

As per possibilities, the traditional ways of starting machines are (roughly): 1) bare metal, 2) subsets of bare metal, or 3) using relatively dedicated software to start virtual machines. Starting a virtual machine from a web browser is new way of starting virtual machines, with a very common denominator.