Issue #88587 | webcompat.com

Browser / Version: Firefox 94.0
Operating System: Mac OS X 10.15
Tested Another Browser: Yes Edge

Problem type: Site is not usable
Description: Browser unsupported
Steps to Reproduce:

Go to https://boxy-svg.com/app

Expected:
Use the SVG editor

Actual:
Redirected to https://boxy-svg.com/requirements

This happens in https://boxy-svg.com/runtime/web-preload.js

which contains

(function() {
  var isSupportedBrowser = (
    window.customElements                        !== undefined &&
    window.customElements.define                 !== undefined &&
    window.requestIdleCallback                   !== undefined &&
    window.Animation                             !== undefined &&
    window.CSSStyleSheet                         !== undefined &&
    window.CSSStyleSheet.prototype.replaceSync   !== undefined &&
    window.DOMPoint                              !== undefined &&
    window.DOMRect                               !== undefined &&
    window.HTMLDialogElement                     !== undefined &&
    window.HTMLDialogElement.prototype.showModal !== undefined &&
    window.MutationObserver                      !== undefined &&
    window.PointerEvent                          !== undefined &&
    window.ResizeObserver                        !== undefined &&
    window.ShadowRoot                            !== undefined &&
    window.CSS                                   !== undefined &&
    window.CSS.supports("color", "var(--test)")                &&
    window.CSS.supports("d", "path('M 0 0 H 10')")
  );

  if (isSupportedBrowser === false) {
    window.location.replace("/requirements");
  }
  else {
…
}

The part which fails is

window.CSSStyleSheet.prototype.replaceSync which is behind a pref in about:config on Firefox Nightly 94.

This fails too in Safari Tech Preview, which has no support at all.
For Firefox https://bugzilla.mozilla.org/show_bug.cgi?id=1613746

View the screenshot

Browser Configuration

gfx.webrender.all: false
gfx.webrender.blob-images: true
gfx.webrender.enabled: false
image.mem.shared: true
buildID: 20210930095024
channel: nightly
hasTouchScreen: false
mixed active content blocked: false
mixed passive content blocked: false
tracking content blocked: false

View console log messages

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