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The HTTP spec draws such a distinction, albeit implicitly, in the form (and name) of its concept of "user agent."

Over time it degraded into declaring compatibility with a bunch of different browser engines and doesn't reflect the actual agent anymore.

And very likely Perplexity is in fact using a Chrome-compatible engine to render the page.

The header to which you refer was named for the concept.

user agent = which bullshit css hacks and js polyfills will be needed