The United Kingdom and European Union are making progress in their negotiations on Britain's membership in the Continental bloc, but a number of important questions have yet to be answered. On Tuesday, European Council President Donald Tusk sent a letter to each of the European Union's 28 members. The document contained a series of proposals aimed at reaching a compromise with London on its terms for remaining in Europe. But even though most of the proposals were in line with British demands, there's no guarantee that they will smooth the path toward a final deal.

Tusk's letter included several ways of protecting the national sovereignty of EU members, especially those that are outside the eurozone. He suggested that the bloc accept the fact that "different paths of integration" are available to EU countries, and that it should "not compel all member states to aim for a common destination." However, Tusk stopped short of advocating the removal of the "ever-closer union" concept that has traditionally described the bloc in its treaties, as British Prime Minister David Cameron had requested.

Still, these sentiments indicate that the European Council's chief is willing to view the European Union as an organization whose members have different long-term goals when it comes to integration. While this is more the acknowledgement of a reality that has existed for some time, it is also an unusual admission on Brussels' part, one that could open the door to future legal challenges to Continental integration. Of course, Tusk's new interpretation of the nature of the European Union will only be formalized in European treaties the next time they are negotiated. And, because no renegotiations are currently on the horizon, it is unclear what will happen in the meantime.

One of Tusk's proposals also grants EU members additional powers to veto legislation the European Commission brings to the table. But the process of vetoing will not be easy, since it would require the support of 55 percent of member states' parliaments to block a commission plan. This setup means that London would have to negotiate with a number of governments every time it wants to strike down a new EU law. The system would not be retroactive, so Britain could not use it to modify existing EU norms.

At the same time, Tusk's plan promises to protect the interests of countries outside the eurozone in the event the currency integration area expands. However, the steps he would take to do so are vague at best. According to Tusk's letter, if an unspecified number of non-eurozone states express concern about projects involving the eurozone, the president of the Council of Ministers must address their worries. But this is not the same as giving non-eurozone countries veto power over decisions concerning the currency union. 

Finally, Tusk recommended that the European Union introduce an "emergency brake" mechanism that would allow members to deny employment benefits to EU citizens working in their borders for up to four years. According to Tusk's plan, such safeguards would enable EU countries to protect their welfare systems from the strain of excessive migrant influxes. This is a notable development because, if passed, it would allow EU states to discriminate against workers from other member states for the first time. But it is still unclear just who would decide if an emergency situation exists — and how that would justify limiting foreigners' benefits. It is also unclear how Britain's Euroskeptics will respond to this plan.

Cameron has repeatedly vowed to reduce the number of EU migrants within Britain, and he has made it clear that he considers the United Kingdom's generous benefits to be a factor pulling more migrants to the country. Though Tusk's proposal would give London new tools with which to limit foreigners' access to those benefits, it is still a long way from Cameron's original demand for a complete four-year ban on employment benefits for non-British workers. It is also unlikely to have a big impact on immigration; EU workers would still flock to the United Kingdom's labor market, which boasts an unemployment rate that is about half the EU average. At the same time, countries in Central and Eastern Europe will probably oppose even Tusk's version of the reform: Many of their citizens emigrate to the United Kingdom, and the new proposals could restrict their migrant rights.

And so, while the latest letter is a significant development in Britain's talks with Europe, the negotiations are far from over. Tusk's proposals have merely opened the door to more difficult debates. At home, Cameron will have to convince his government — and his people — that Tusk's promises are enough to justify Britain's decision to stay in the European Union. In the coming days, London will probably seek to clarify the many gray areas that remain. However, one thing needs to be kept in mind: Most of the reforms Tusk is suggesting will only be implemented after the British referendum has already taken place, meaning voters will have to base their decision on promises — no matter how binding — rather than facts. With opinion polls still showing contradictory results, it is entirely possible that Britain could sign a deal with the European Union and then be forced to leave the bloc by its citizens.

Meanwhile, support from the European Union's other 27 members is also far from certain. Northern European countries will likely back most of the proposals, but the situations in Southern and Eastern Europe will be much more complex. The bloc's members are scheduled to discuss Tusk's ideas during a Feb. 18-19 summit, though it is unlikely that talks will end there.

Tusk, for his part, was hoping to strike a balance with his letter. On one hand, he hoped to give Cameron the concessions needed to frame the deal as a political victory and campaign at home in favor of staying in the European Union. On the other, he tried to make the reforms complex and vague enough to avoid creating a dangerous precedent that could further fragment the Continent. All parties involved — Britain, EU institutions and member states — will likely push for amendments to the existing proposals in the coming weeks. One could make the case that only a handful of EU countries are as influential as Britain, and that its attempts to renegotiate its relationship with the bloc will be an isolated case. But even in the likely scenario that Cameron gets only a toned down version of his original demands, there will be little standing in the way of other member states making their own demands in the future.