:startup_css feature breaks with Waterfox, Pale Moon and other browsers

Summary

Sometime since today (2020-08-21), gitlab.com is not displayed properly with Waterfox anymore (main page content is empty>.

This seems to be caused by b4691f52 where the inline "onload" handler is replaced. Waterfox does not seem support this.

After removing the media="print" attribute within the Webdevelop Inspector, page is displayed as expected.

Steps to reproduce

Visit https://gitlab.com/gitlab-org/gitlab/ with Waterfox Classic

Example Project

What is the current bug behavior?

Main content is missing/not visible

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)