right on! I used https://bugdays.com/gzip-base64 to go back and forth.

base64 generates slashes, so the site (and I) run encodeURIComponent in the devtools on the resulting base64 to make sure it's completely url-safe.

---

the poc "payload" is

eNqzKU4uyiwosUvJTy7NTc0r0UtPLXHNSQUxi50qnXMSi4v9EnNTNdRzMtMzStQ1ow1i9YpSc%2FPLUjU0bfShmrm4lBVKMjKLFYAoKTEFACeDHYg%3D

which uri-component-decodes to:

eNqzKU4uyiwosUvJTy7NTc0r0UtPLXHNSQUxi50qnXMSi4v9EnNTNdRzMtMzStQ1ow1i9YpSc/PLUjU0bfShmrm4lBVKMjKLFYAoKTEFACeDHYg=

which un-base64+gzip's to (using the site I posted above):

  <script>document.getElementsByClassName('light')[0].remove()</script>

  # this is bad