Check the operating system first
Windows and macOS can disable HDR for a display even when the panel supports it. Start there before blaming the browser.
External monitor caveats
Some cables, docks, adapters, or ports cannot carry the mode you expect. A monitor may fall back to SDR or lower refresh when the path is limited.
Browser differences
Browser support for HDR signalling can vary. Compare Safari, Chrome, Edge, and Firefox if the result seems odd.
Capability versus quality
HDR support does not prove high brightness, good local dimming, or accurate tone mapping. Pixy reports capability, not a lab HDR score.