User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

See the conversation here (sorry for the indirection, but otherwise this would only be copying a lot of content):
https://gitlab.com/inkscape/inkscape/-/issues/1932

Actual results:

In Firefox, the arrows do not appear.

Expected results:

In Chrome, the SCG markers (arrows) rendered as expected. Untested personally, but according to others on that thread this also works as expected in Safari. Of course, Inkscape shows the arrows as well.

It's arguable this is a defect; feel free to recategorize as needed.