Security vulnerabilities in JavaScript have been used to [deanonymize Tor Browser in the past](https://arstechnica.com/information-technology/2013/08/attackers-wield-firefox-exploit-to-uncloak-anonymous-tor-users/).
It is impossible to hide to the websites that you visit that you are using Tor, because the [list of exit nodes of the Tor network is public](https://metrics.torproject.org/exonerator.html).
The anonymity provided by *Tor Browser* works by making all users look the same to make it impossible to know who is who among the millions of users of *Tor Browser*.
Tails tries to make it as difficult as possible for websites to distinguish Tails users from other users of *Tor Browser*. If it was possible for a website to determine whether you are a user of *Tor Browser* inside or outside of Tails, this would provide a little bit more information about you and reduce your anonymity.