in Activism

Mirrors and Prisms: robust site-specific browsers

Mozilla (amongst others, see Chris Messina’s writeup of the trend, also Matt’s) have been exploring site-specific browsers through their Prism project. These combine aspects of the Web and Desktop environments, allowing you to have a desktop app tuned for browsing just one specific Web site. Prism is an application which, when run, will generate new per-site desktop applications. Currently it does not yet have a fancy packaging/installer, so users will need to install Prism plus the site files separately.

I have started to look at Prism as a basis for accessing robust, mirrored sites, so that a single point of failure (or censorship) might be avoided. With a lot help from Matt and others in #prism IRC chat, I have something almost working. The idea is simple: hack Prism so that the running browser code intercepts clicks and (based on some as-yet-undefined logic and preferences) gets the page from a list of mirrors, which might also be fetched dynamically from the ‘net.

I should also mention that one motivation here is for anti-censorship tools, to give users an easy way to access sites which might be blocked by their IP address or URL otherwise. I looked at FoxyProxy as an option but for site-specific robustness, running a full proxy server seems a bit heavy, compared to simply duplicating a set of files. Here’s what the main Prism app looks like:


Screenshot showing Prism config settings for a site-specific browser.

Once you have Prism installed, you can hack a file named webrunner.js to intervene when links are clicked. In OSX, this can be found as /Applications/

Edit this: _domActivate : function(aEvent)

I added the following block to the start of this function:

var link =;
if (link instanceof HTMLAnchorElement && !WebRunner._isLinkExternal(link)) {

The idea here being that we intercept clicks, and rewrite them to point to equivalent http:// URIs elsewhere in the Web. As far as this goes, it works as advertised. But what I have is far from working… it would need some code in there to find the right mirror URLs to fetch from. Perhaps a list might be fetched on startup or first time a link is followed. It could also do with some work on packaging, so that this hacked version of Prism plus some actual site-specific browser config can be made into an easy-install Windows .exe or OSX .app. For a Windows installer, I am told that NSIS is a good place to start. You could also imagine a version that hid the mirrored URLs from user’s view. Since Prism has a built-in option to completely hide the URL navigation bar, I didn’t investigate this idea yet.

OK I think I’ve written up everything I learned from the helpful folks in IRC. I hope this repays some karma. If anyone cares to explore this further, or wants to help target student projects on exploring it, please get in touch.

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  1. I’m a big fan of, a site-specific browser based on Safari, in part because of all of the following features:

    Fluid includes Tabbed Browsing, built-in Userscripting (aka Greasemonkey), URL pattern matching for browsing whitelists and blacklists, bookmarks, auto-software updates via the Sparkle Update framework, custom SSB icons, a JavaScript API for showing Dock badges, Growl notifications, and Dock menu items, and more.