And then there is https://schema.org/ It's the item* attributes, e.g.: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/... Also Dublin Core in <meta> tags. Why do they keep adding conflicting meta data formats to HTML!?!

They don't conflict; they were designed to work together. You can have schema.org (in JSON-LD, RDFa, or micro data) on the same page as Dublin Core, etc.

For example, there's no explicit property in schema's Person type [1] for a nickname. But the FOAF standard does [2].

Just add FOAF to the JSON-LD context:

    {
      "@context": {
        "@vocab": "https://schema.org/",
        "foaf": "http://xmlns.com/foaf/0.1/",
        "pronouns": "https://schema.org/pronouns" 
    }

    "@type": "Person",
      "givenName": "Timothy",
      "familyName": "Berners-Lee",
      "foaf:nick": "TBL",

[1]: https://schema.org/Person

[2]: https://xmlns.com/foaf/spec/#term_nick

I think if you are using Dublin Core, it’s because you’re a library. Maybe I am off the mark, but that is the sense I get from this—not all these standards should be used for all pages on the web.

I think you should just think about what metadata you actually care about, and the main metadata I care about (choose your own list) is authorship, publish date, last update, subject keywords, thumbnail (OpenGraph 1200x630), and summary.

There’s a long list of additional metadata that I could put in my webpages because there are standardized ways to do it, but, why bother?

To be fair schema.org and dublin core say “when a property is name ‘title’ it means …” and you can expect to find the following properties…

Json-ld says: if you want to know whether the “title” property means the schema.org or the dublin core variant then you can find out which it is by <json-ld algorithm>

So you’d always use json-ld _with_ schema.org or something.

There is also microformats.

https://xkcd.com/927/