Silver-bordered cards are not tournament-legal and are not covered by the comprehensive rules.
Usually silver-bordered cards are printed in Un-sets (Unglued, Unhinged, and Unstable). However certain promos like the Holiday cards and the HASCON promos also use a silver border.
They feature mechanics that would be impossible to print in a normal expansion.
From the glossary of the Comprehensive Rules (Guilds of Ravnica (October 5, 2018))
- Cards in certain sets and certain promotional cards are printed with a silver border. Silver-bordered cards are intended for casual play and may have features and text that aren’t covered by these rules.