Nevertheless, I want to make this editor. How?

They are using the ace editor: http://ace.ajax.org/

There's some bootstrap and other jazz, and obviously a server-side component to perform the LaTeX rendering

Thanks! What are they using for the PDF viewer?

pdf.js: https://github.com/mozilla/pdf.js

NOTE: If you do decide to build your own thing, I think it would be cool if you could run LaTeX in the browser directly (I guess you could do this by building pdflatex with emscripten)

Your guess is 100% correct: http://manuels.github.com/texlive.js/website/ (Discussion: https://qht.co/item?id=5083361)

Works in chrome for mac, but falters on iPad (even though http://caniuse.com/#search=worker says webworkers are supported)

Thank you. Do you know how they did the "intros" -- the dark overlay with popups that point out various buttons / explain concepts? Are they using intro.js, something else, or something they made themselves but haven't released yet?

Also, could you provide more detail about what you mean by "run LaTeX in the browser directly"? It sounds like a fun challenge.

Intros: https://github.com/jeff-optimizely/Guiders-JS

"run LaTeX in the browser directly": Right now it appears to send data back to their servers to compile every time. What would be cool, especially for offline work, is a pure-JS implementation of the rendering engine.