what's data-reactid attribute in html?

Question

While I was going through the HTML of some pages, I noticed that some of them use this attribute \"data-reactid\" like :

Answer 1

That's the HTML data attribute. See this for more detail: http://html5doctor.com/html5-custom-data-attributes/

Basically it's just a container of your custom data while still making the HTML valid. It's data- plus some unique identifier.