There is not really a need to modify LaTeX at all to make it run in the browser. It already exists. Without modifying a single line of code, we have implemented a full browser-based port of LaTeX as part of our Browsix project, which makes it possible to run full, unmodified Unix applications inside the browser. See http://browsertex.org and http://browsix.org (and http://bpowers.net and https://jvilk.com/ and http://plasma.cs.umass.edu).