Why doesn't execcommand work?

Question

JavaScript

execCommand doesn't work. I can't figure out why the text isn't getting bold. Maybe I'm doing something wrong. The code itself:

<span onclick="document.execCommand('bold', false, false);">h1111</span>
<div class="_3t4g5rwe34t3" contenteditable></div>

Link to example: https://jsfiddle.net/nge37Lw1/