In theory, in 1996 Netscape and Microsoft agreed to kill <blink> and <marquee> <https://www.w3.org/People/Raggett/book4/ch02.html>, but although they were kept out of the spec, neither removed its implementation, and then IE dominated the browser market, and <marquee> became popular enough that the remaining parties were bullied into shipping it (Netscape in 2002, Presto in 2003, no idea about the KHTML/WebKit timeline), and so ultimately it was put into the HTML Standard.