Sites like Google are notoriously opaque about how they handle the search rankings that are crucial for many businesses. Meanwhile, companies like Spotify have complained about having to give Apple a cut of sales to appear in its App Store, while at the same time competing directly against Apple Music.

If the proposed rules are passed by the EU parliament, Google or Bing would have to tell customers exactly why they dropped their rankings. If a situation can't be resolved amicably, the platforms would be required to appoint mediators and pay at least half the costs. They would also need to explain how they treat their own services compared to third-party businesses that provide identical products (as is the case with Apple Music versus Spotify, for example).

The EU has been moving toward regulation because tech sites don't have a good track record in policing themselves. Last year, for example, the EU fined Google $2.7 billion for manipulating search results to promote its own products, to the detriment of other e-commerce sites. "Platforms and search engines are important channels for European businesses to reach consumers," said EU Commissioner Mariya Gabriel, "but we must make sure they are not abusing their power, and thus bringing harm to their business users."