To what JLPT (or CEFRL) level do NHK Web Easy articles correspond?
Based on this article from NHK放送技術研究所:
ニュースのやさしい日本語では、旧日本語能力試験の3級と4級の語彙と文法の範囲に収めることを目指しましたが、語彙の制限は緩めました。また必要に応じて難しい文法も使います。このため、NEWS WEB EASYは3級と4級の初級の日本語能力では完全に理解できず、中級準備レベルの外国人が理解できるニュースだと考えています。
Paraphrasing it, the goal is was to stick to the vocabulary and grammar of the old JLPT 3/4. However, especially with regards to vocabulary they often used harder words and because of that beginners who are JLPT 3/4 cannot fully understand it. But they believe intermediate learners can understand it.
They also did a survey on the percentage of text understood in the "easy" news compared to the "non-easy/original" news by people who passed JLPT 1,2, and 3. The first point in the graph is the original text and the second point is the easy text. In all cases we see an increase in the understanding. They thus conclude that
やさしい日本語のニュースでは、ほぼ全内容を理解できたと言えます。 It can be said that (people who passed JLPT level 3) can understand nearly all the material in Easy Japanese News.
For comparison, they also surveyed Japanese school children.