The site is up, but is brand new and still in beta — what mobile OS/Browser combo are you using? We'd love to try and debug your white screen...
Not the parent poster, but I use NoScript on desktop Firefox. Beyond whitelisting observablehq.com and observableusercontent.com I found that the content was completely missing until I also unblocked objects from fonts.gstatic.com. Could be a clue.
Ah, this could be the case. I block external fonts but never had a problem because of this.