Apple is working on a Dark mode user preference for individual websites based on references found in open-source WebKit code.
The new option, spotted by 9to5Mac, is called "Overriding the system color-scheme with a per-website preference" and is referred to in the GitHub -hosted WebKit code.
Since Apple introduced Dark mode for the iPad and iPhone, developers have updated their app interface to match the system setting or allow users to override it. Users would be able to correct sites that do not display properly in either Light or Dark modes with the new per-website preference.
The per-website display setting is one of the options Apple is developing to block popups on specific websites. To dismiss a popup, tap a Cancel or other button and it will look like a system alert. Cookie consent is a process that many websites use.
It is not clear whether the new features will be available to use in the future when the new operating system versions are released, or later in the year when the new features are announced.