How to position DivElement

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How do I position a DivElement?
I want one in the middle of the screen, and one at the bottom.

This CSS does not work.
The DivElement is centered on it’s top left corner, but the left and top don’t work.
position: absolute;
z-index: 9999;
left: -200px;
top: -100px;

You should use positive values. Working example here!

position:absolute;z-index:9999;
width: 200px;
height: 100px;
background-color: blue;
top: 100px;
left: 200px;

Ok it doesn’t work with negative values I see.
But how can I center the div?