If websites are no longer able to tag users with cookies — the small bits of computer code they use to track internet surfing — the knowledge of the browsing history and consumer profiles of the people visiting their sites could rest solely with the companies whose services users are continuously logged in to.

This is bad news for online ad-buying companies — demand-side platforms and agency trading desks like GroupM's Xaxis — which use cookies to place bids on online advertising slots on behalf of their clients. Without cookies, they might only be able to get this data from Facebook, Google, Microsoft, or Apple.