No major browser ever shipped with a Tcl interpreter as far as I can remember. By major I mean Netscape and IE. All of them had a JavaScript interpreter. It was a battle that never happened because it couldn't be fought. Maybe there was a chance in server side software, where JavaScript has not been an option until Node.js. Java and PHP won the server post 1995 and Perl did it before them. Desktop side, the browser + HTML + JavaScript won the desktop, served from either Java or PHP or ASP, then from one of the other post 2005 stacks.
Not with the browser, but there was a TCL runtime you could download to run TCLets from web servers. I had a few contracts for that; some intranets made heavy use of it.
There is/was a plugin for client side Tcl, the Tcl/Tk Tclet Plugin, originally released during the 1990s. It filled the same role as Javascript; but never really gained popularity.
