The colors are meant to be within a calibrated (sRGB) color space. Images and videos can request to be in a different color space which includes the HDR range. CSS can also request colors outside that space, but that is done by using extended RGB triples (e.g. RGB(999,999,999) for ultra white).

The difference is while most things support sRGB, those other color spaces may just be outside what the display can handle. My Ultrafine 5k for instance does not show a discernible difference between the two QR codes.

You also have the issue that static images displayed at higher brightness will use more power and require quicker mitigations to prevent burn-in, so an 'ultra white' background may just not be something supported for a web page.