Practically any profitable i.e.sustainable web site which does not sell goods or services but serves users who use the site for free must have much more than 50 000 users.
So this law applies to even the smallest non-local businesses.
Feature testing is almost always a better way to go
We are using a huge amount of javascript, and working around browser bugs is a regular activity. About every third minor update of a browser has a new bug/regression which affects us. In these cases only the user agent string helps. A fix for a few Chrome versions for example breaks both older and newer Chrome versions, and maybe the same regression come back a year later. However, I also understand Vivaldi developers. It is a shame of web developers, especially of popular web sites, if they abuse the user agent.
We have unusually complex one page webapps. If I do not count the various browser bugs which appear regularly (and reapper again, seriously), then the remaining 90% of mysterious bugs user experiencing are caused by ad-blockers and virus killers, and in a less extend other browser extensions.
We have to write various checks to test if the code running in the browser is actually our Javascript code, or one of these miscreants modified the code (and break it).
Machines have less problems. I'd like to be a machine. -- Andy Warhol