because card reader support is still shit in browsers in 2026
Around a decade ago I was working at a company that used smartcard login for authenticating to internal sites. I've heard of many others doing the same. USB card reader worked fine in both IE and Firefox at the time, so I take your statement to mean that we've somehow regressed since then (not surprising) or this was an isolated instance of success (less likely, considering the US government also uses this: https://en.wikipedia.org/wiki/Common_Access_Card).
There are two parts to this. The basic standards based auth stuff "sort of" works. Everything else requires a browser plugin. One of the Debian dudes (of grep.be fame) maintains a Linux version that works in many cases, but for some reason many non-government organisations require the use of a different plugin, one that only works on Windows and mac.
As an aside: signing things has a particularly awful UX. I never know what I'm signing, I have no way to verify that what's on the screen is what's being signed. And then there are orgs that use eID based PDF signing, which again requires different plugins. In short: a shitshow.