I think HTML -> Markdown is a bit of a red herring.

In many cases, a Markdown distillation of HTML can improve the signal-to-noise ratio — especially for sites mired in <div> tag soup (intentionally or not). But that's an optimization for token efficiency; LLMs can usually figure things out.

The motivation behind Sosumi is better understood as a matter of accessibility. The way AI assistants typically fetch content from the web precluded them from getting any useful information from developer.apple.com.

You could start to solve the generalized problem with an MCP that 1) used a headless browser to access content for sites that require JS, and 2) used sampling (i.e. having a tool use the host LLM) to summarize / distill HTML.