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The bottom of the Wikipedia "talk" page says that Microdata is dead, and superseded by rdfa and json-ld.
What is the difference between the formats (or value), and which one should be used?
I'm mostly interested in SEO for Google Shopping, and contact sharing.
Microdata is not dead. While Microdata will not become a W3C standard¹, it’s still part of WHATWG’s HTML Living Standard.
(in the context of HTML documents)
Microdata and RDFa are conceptually similar: both syntaxes define attributes that get added to your existing HTML elements.
JSON-LD is a syntax that gets added to a script element (used as data block, not as script), separately from your existing markup.
So the primary difference between Microdata/RDFa and JSON-LD is that you have to repeat the content if using JSON-LD, while you can² mark up your existing content if using Microdata/RDFa.
These three snippets convey the same structured data (a Person with a name of "Alice"):
<!-- Microdata -->
<p itemscope itemtype="http://schema.org/Person">
Hi, I’m <span itemprop="name">Alice</span>.
</p>
<!-- RDFa -->
<p vocab="http://schema.org/" typeof="Person">
Hi, I’m <span property="name">Alice</span>.
</p>
<!-- JSON-LD -->
<p>
Hi, I’m Alice.
</p>
<script type="application/ld+json">
{
"@context": "http://schema.org",
"@type": "Person",
"name": "Alice"
}
</script>
JSON-LD if you want to add a blob of structured data, without having to care about existing markup. Might be easier for tools that don’t support structured data (and you use other tools to generate/add it), or when copy-pasting ready-made snippets.
Microdata/RDFa if you want to make use of your existing markup (so you don’t have to duplicate the content → DRY). Might be easier if adding it by hand, or when tools already support it (like Drupal).
If deciding between Microdata and RDFa: I recommend to use RDFa instead of Microdata, primarily because of the differences outlined in this answer on Stack Overflow.
RDFa and JSON-LD are both W3C Recommendations and both are RDF serializations.
For reference: an older related answer about Microformats vs. Microdata vs. RDFa / Schema.org.
¹ At least not in the foreseeable future. https://www.w3.org/TR/microdata/ became a W3C Working Group Note in 2013 (https://www.w3.org/TR/2013/NOTE-microdata-20131029/), so it no longer gets updated/edited, and it will not become a W3C Recommendation.
² Of course Microdata/RDFa would allow to do something similar to JSON-LD: instead of marking up existing content, you could add meta/link elements that repeat the content. Bad practice, but possible.
