I recently tested a language-learning site that used live frontend filtering to block HTML input (e.g., <img> <svg> tags were removed as you typed).

But by injecting the payload directly via browser console (without typing it), the input was submitted and stored.

Surprisingly, the XSS executed later on my own profile page — indicating stored execution from a DOM-based bypass.

I wrote a short write-up here:

https://is4curity.medium.com/xss-before-submit-a-dom-based-execution-flaw-hidden-in-plain-sight-5633bdd686c9

enjoy