title: what's data-reactid attribute in html?
body: <p>While I was going through the HTML of some pages, I noticed that some of them use this attribute "data-reactid" like : </p>
  
  <pre><code> &lt;a data-reactid="......" &gt;&lt;/a&gt;
  </code></pre>
  
  <p>What is that attribute and what is its function ?  </p>