The problem is that spans are inline elements. Just add display:block; to your CSS and it will fix the problem.
$(myspan).focus();#myspan {
border: 0;
outline: 0;
display: block;
}<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<span id="myspan" contenteditable=true></span>