The phrase "unreasonable effectiveness" is used here in the context of AI. Where "we put a whole bunch of markup characters in the context" not messing up the output significantly could be considered unexpected. The actual performance impact is unknowable, given the difficulty of measuring LLM performance. And whomever pays for your tokens won't be pleased either.

While it has been used in the context of HTML before, it's a bit of a "meme" as several papers used that phrase. Much like the "X is all you need" snowclone.

The HTML version dates from 2021 (https://shkspr.mobi/blog/2021/01/the-unreasonable-effectiven...) when client side rendering and web apps were still their peak.