[tech] [spec] Decide on use of URL fragment

[nervuri]

> Finally, the fragment may also be used for other purposes. For instance, a hash could be used by the browser to check if a page has been modified, or if a file has been downloaded correctly. So it might be wise to use prefixes, like #find= and #hash:sha256=.

Two more quirky ideas for alternate uses of fragments:

* content-length (#size=1436) - can be added automatically in a server-generated file listing. Optionally, a Gemini server could append #size= to all links pointing to domains which it hosts. And browsers could limit their use of #size to instances where the destination file is hosted on the same domain as the page that links to it.

* certificate fingerprint (#cert:sha256=EB07E03...) - embeds a server's (or a user's) perspective on the cert used by a linked-to domain. Can be periodically refreshed by the server. If it doesn't match, browser shows a warning.