  In the early days of the web, giving the same web page to different browsers
could provide very different results. As the rendering engine behind each
browser would differ, some elements of a page could break or be positioned in
the wrong location. At that time, the User Agent (UA) string was introduced for
content negotiation. By knowing the browser used to connect to the server, a
developer could provide a web page that was tailored for that specific browser
to remove any usability problems. Over the past three decades, the UA string
remained exposed by browsers, but its current usefulness is being debated.
Browsers now adopt the exact same standards and use the same languages to
display the same content to users, bringing the question if the content of the
UA string is still relevant today, or if it is a relic of the past. Moreover,
the diversity of means to browse the web has become so large that the UA string
is one of the top contributors to tracking users in the field of browser
fingerprinting, bringing a sense of urgency to deprecate it. In this paper, our
goal is to understand the impact of the UA on the web and if this legacy string
is still actively used to adapt the content served to users. We introduce
UA-Radar, a web page similarity measurement tool that compares in-depth two web
pages from the code to their actual rendering, and highlights the similarities
it finds. We crawled 270, 048 web pages from 11, 252 domains using 3 different
browsers and 2 different UA strings to observe that 100% of the web pages were
similar before any JavaScript was executed, demonstrating the absence of
differential serving. Our experiments also show that only a very small number
of websites are affected by the lack of UA information, which can be fixed in
most cases by updating code to become browser-agnostic. Our study brings some
proof that it may be time to turn the page on the UA string and retire it from
current web browsers.
