well-known

 Well-known  refers to designating a common URL on domains for data to be located typically automatically by some software (like a browser or a search engine), and is generally an antipattern to be avoided, because it breaks the portability of content across directories and systems; see follow your nose instead.

"Well-known" are prescribed URLs/paths, whereas there are also emergent common page URLs published by IndieWeb sites like about, contact, etc. See:
 * page

There are a couple of common patterns or categories of "well-known" URLs:
 * 1) a root level file
 * 2) stuff inside a root level "/.well-known/" directory, standardized by https://www.rfc-editor.org/rfc/rfc8615

Root Level
Historically there are several root level paths that are used by browser and search engines, and it's unlikely that we’ll ever be rid of them:
 * robots.txt
 * favicon.ico

However there has been a disturbing pattern of new proposals in form of something.txt over the years:

something.txt
(if this list gets too big, create a separate page)
 * ads.txt
 * Criticism:
 * humans.txt
 * : Criticism: I set up one of these years ago then forgot about it. It was last updated 2011-05-08 and I rediscovered it 2023-05-25, demonstrating the sidefile-antipattern. I finally removed and redirected https://gregorlove.com/humans.txt to my about page.
 * security.txt
 * 2020 trust.txt


 * ai.txt
 * Announced 2023-05-30 and used by their own API to "communicate the permissions set by ai.txt files to our growing network of AI researchers and partners, including Hugging Face and Stability AI." and on Twitter
 * Their site fails to implement it themselves, https://site.spawning.ai/ai.txt returns a 404

API endpoints
Apparently these root files are a convention on some sites: Per:
 * /code.json
 * /data.json
 * /sellers.json
 * https://twitter.com/mistersql/status/1528833918245511171 (also yet another mislabeling of 'microformats' for something they are not)
 * "did you know many gov't sites have a code.json or data.json at the root of their domain to let you discover the opensource and open data they have available? https://www.dol.gov/code.json https://www.dol.gov/data.json  I haven't found if there are any more of these microformats" @mistersql May 23, 2022

inside a root well-known directory

 * Let's_Encrypt (option)
 * webfinger