This is a web based measurement instrument for detecting some forms of web based censorship.
It is not meant to be a full replacement of OONI Probe on desktop or mobile, but rather complements these datasets with an approach that is simpler to run for end users.
Prior work exists on how to measure censorship from the web browser. The first appearance of this method is credited to Dan Kaminsky with CensorSweeper. This methodology was further expanded upon by Sam Burnett et. al with a tool called Encore.
At the base the idea is pretty simple, you load a bunch of invisible (or
visible in the case of CensorSweeper) img, iframe or script tags inside
of the measurement web page and register the load or error event listeners.
The load event will tell you if the resource in question is accessible, while
the error event will tell you if it's being interfered with.