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What an Onion-Location header says about a Torzon onion
Tor Browser can show a purple ".onion available" button when a site advertises its onion address.
How it works
A regular website can send an Onion-Location header naming its onion address. Tor Browser then offers to switch.
Why it is useful
If you trust the regular website, the header is a convenient way to get the matching onion address without retyping it.
Its limits
The signal is only as trustworthy as the site sending it. A copied website can advertise a fake torzon onion just as easily.
Most onion-only services
Services that have no regular website never send the header, so its absence says nothing either way.
An Onion-Location button is only as trustworthy as the website that shows it.