Just checked, the "Immersive Reading Mode" in Edge works fine on the page mentioned.

Works for me on that page.

Does this URL work ?

about:reader?url=https://www.toptal.com/qa/how-to-write-testable-code-and-why-it-matters

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Hm it works for me now as well. Must've been fixed at either end.

Btw, I'm not entirely sure how hardware acceleration could affect this... I'm interested to know how you got there.