How to test an HDR monitor in your browser

Start with the setup, since most HDR problems are settings. On Windows, turn on Use HDR in Settings, System, Display. On a Mac, built-in XDR screens show HDR on their own, and an external HDR monitor needs High Dynamic Range turned on in System Settings, Displays. Then check the monitor itself. Many switch to their HDR mode when an HDR signal arrives, but some have an HDR setting in the menu, and eco or power saving modes can cap brightness.

The browser matters too. Chrome, Edge and Safari 26 or newer show HDR images. Firefox doesn't yet, so every patch there looks like SDR white. The Your setup box on the tool says whether your screen reports HDR and whether your browser can show HDR images, which rules out the two most common problems before you start.

Now run the test. Press Start fullscreen, then hide the controls with H or a tap, since their box adds light of its own. The first patch is plain SDR white. Each patch after it asks for more light, from 203 nits up to 10,000. Find the last one that's still brighter than the one before it. Past that point the patches look the same, because your setup has nothing more to give.

The labels are what each patch asks for, not a reading. The browser and OS tie HDR to your SDR white level, so moving the SDR content brightness slider in Windows moves every patch with it. For real nit numbers you need a colorimeter, or a review that used one.