Captive portal : external splash page specs?

Captive portals - like in restaurants, hostels, airports, ... - are detected by OSes. When detected, OS displays a browser-like windows (~popup). (Generally in order to ask for authentification, login, credits, or simply displaying conditions or warning).

I simply wonder what are the specification of such a "splash page". Example : iOS detects a captive portal, allright, it looks for the splash page, but where does iOS find it ?

Solution 1:

There is no specs anywhere. I phoned Apple and got nothing more.

I decompiled CNA.app and got nothing interesting.

I digged the Console and got some infos.

Absolutely opaque.