What law? The U.S.'s Section 508 and other laws and regulations based on WCAG, don't require that sites honor `prefers-color-scheme` or to work correctly when `forced-colors` is active.

Nevertheless, it's a good idea to do so (though if a site honors `prefers-color-scheme` it should also have a setting so the user can choose to have that site not match their system's color scheme setting).