Stylish, a web browser extension that allows for users to incorporate custom-made user interfaces on a per-website basis, has been described by software engineer Robert Heaton as "a covert surveillance tool disguised as an outstanding browser extension."
In that same post, Heaton goes on to describe a timeline of how Stylish has had tracking tools implemented directly into the extension. The Google Chrome release of the extension had this tracking capability included since January of 2017, while the Mozilla Firefox release has had it since March of 2018. The original creator of Stylish sold the extension in August of 2016, and the extension was then sold again in January of 2017 to SimilarWeb, known for its data aggregation and analysis.
As Stylus's primary objective when it was created was "to remove any and all analytics, and return to a more user-friendly UI," it is an otherwise faithful alternative to Stylish, being based off of the source code of the last version that the original developer of the software released, 1.5.2.