Currently, host name "localhost.localdomain" resolves to the loopback addresses ::1 and 127.0.0.1, bypassing native DNS, and the corresponding origin is treated as secured. The goal of this entry is to remove this non-standard behavior.
Standards describe an optional resolution of the "localhost" host name and trustworthiness of corresponding origin  . Users have complained about inconsistency between Chromium (which implements the spec), Firefox (which only implemented it recently) and WebKit (where patches are being submitted). Hopefully things can be make consistent and the specification a bit stricter. However, Chromium also has similar but non-standard behavior for "localhost.localdomain". Removing this would help to make things more predictable for users.
Unknown standards status - check spec link for status
Status in Chromium
Consensus & Standardization
Last updated on 2022-01-21