How to get the HTML code of an element?

https://developer.mozilla.org/en-US/docs/Web/WebDriver

I only see commands like the following on the above page. But I don't see a way to get the HTML code of an element.

Get Element Attribute
Get Element Property
Get Element Tag Name

I tried outerHTML. But it does not seem to be there. Do I miss anything?

$ curl -s -H 'Content-Type: application/json' -d '{"url":"http://httpbin.org"}' http://127.0.0.1:4444/session/91ba315a-d796-440f-8e45-35d384fb1c8a/url
{"value":null}
$ curl -s -H 'Content-Type: application/json' -d '{"using":"xpath","value":"//h2[@class=\"title\"]"}' http://127.0.0.1:4444/session/91ba315a-d796-440f-8e45-35d384fb1c8a/element
{"value":{"element-6066-11e4-a52e-4f735466cecf":"74d674fc-7f39-49ee-9a25-5bbafe6aaff9"}}
$ curl -s http://127.0.0.1:4444/session/91ba315a-d796-440f-8e45-35d384fb1c8a/element/74d674fc-7f39-49ee-9a25-5bbafe6aaff9/attribute/outerHTML
{"value":null}

mozilla / geckodriver

How to get the HTML code of an element? #2072

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