It does not apply any dark overlay, at least not on HDR screens, it just looks like it does due to the big difference in brightness.

I would assume the iPad Air 4 doesn't have a HDR screen, but maybe I'm wrong. On HDR capable screens you should still see a huge difference even on maximum brightness.

Apple does some trickery to display HDR on non HDR Apple displays. Look up EDR.

On some post-2017 non-HDR Apple displays, the OS actually dynamically adjusts the screen's gamma curve to simulate this effect. On machines that support this, if you hold Option down while opening the Display control panel, there's an additional option that appears to enable/disable this.

On my 2023 M2 MBP (XDR display) there is still a large difference between the white on the page at max brightness and the video whiteness. In fact the video gets even brighter at max brightness.

Could you try manually increasing the brightness to maximum if that's possible? (Though I suspect with local dimming zones and the language on the Apple website the whole display can't be kept at "max" brightness, as a local zone can still be brighter.)

It's an HDR video and it's the OS that controls how it looks. If your display is HDR (OLED and Mini LED Apple displays) it will brighten just the video pixels. If your display is not HDR, it will do as you describe (across the whole OS, not just the webpage). Apple calls this EDR I believe.

Yeah I think you're right. I thought that my iPad had an HDR display (it's got a wide gamut coverage and can show HDR content) but apparently it's not "Apple HDR".

Though - lots of non-OLED/miniLED displays have HDR... why does Apple have a different standard?

I believe they do it this way because they want to be able to display HDR and SDR content on the same display at the same time.

Because their non-hdr displays don't have enough peak brightness for good looking HDR.

That's.... terrible way to handle it. Make whole screen look worse

Are you on a MBP? It doesn’t actually make anything worse, it raises the screen brightness at the same time as it dims the content, so it stays at the same absolute brightness and the transition is imperceptible.

About 15 years ago I had a monitor from Philips that had the ability to software control the backlight at a reasonably fine level (I don't recall if it was actually the same resolution as the display, but it pretended you could choose it per pixel).

The primary use case of the included software was to highlight the focused window and dim the rest. Other than that, it was more of a chore to use.

On my MBP in Safari, this adjustment is done in several steps. When the page loads, the image is there but takes 6-8 increases before stabilizing at the final brightness. It's not a smooth ramp at all.