It would be almost impossible to enforce. Publishers have a point of contact, an address or a hosting company, some sort of physical place where you can find whoever is in charge of the site. In other words, somewhere to send legal documents and summonses should a government wish to pursue legal action.

This is only partly true for web browsers. Google, Mozilla and Microsoft have addresses. But what all the browsers that have forked from open source projects? If someone forks Chromium and adds nasty features, how do you track them down if they did everything anonymously?

More to the point, if a law is passed that says "all browsers must do X,Y and Z". How to you enforce that in a world where open source is so prevalent? The big players may add the requirements to their flagship browsers, but if those browsers have open source underpinnings they have no control over the forked versions.

It's the publishers who are abusing browser capabilities, its much easier to force them in to compliance rather than trying to legislate how browsers work.