This article is kind of odd in that it implies that HTML inherently satisfies the uniform interface constraint, and somehow JSON can't. JSON can provide a linked representation of the state of the application just like HTML can.
It does mention this in the section "“REST” Wins, Kinda…"
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Some pushed through to Level 3 by incorporating hypermedia controls in their responses, but nearly all these APIs still needed to publish documentation, indicating that the “Glory of REST” was not being achieved.
JSON taking over as the response format should have been a strong hint as well: JSON is obviously not a hypertext. You can impose hypermedia controls on top of it, but it isn’t natural.
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Personally, the section "The Crux of REST: The Uniform Interface & HATEOAS" says it all.
