I'm now doing it this way,which is not centered:
$div = $('<div style="background-color:yellow;position:absolute;top:0;"><b>Loading...</b></div>').appendTo('body');
I'm now doing it this way,which is not centered:
$div = $('<div style="background-color:yellow;position:absolute;top:0;"><b>Loading...</b></div>').appendTo('body');
For auto margins to work you also need to define a width for the div as well, else it just takes up the whole screen width and you'll see no difference.
However, in this instance all you need is to add text-align: center;
to your original CSS as far as I can see?
<div style="text-align: center">
<div style="text-align: left; margin: 0 auto; width: 50%; background: yellow">
Your text here
</div>
</div>