How to Position an Element Absolute from Center - CSS

If you know the width of the advertisement then you can do this:

position:absolute;
left:50%;
margin-left:-250px;
width:500px;

notice the negative margin-left which is half of the width of the element to be positioned

If you want to center something and keep it responsive width/height then you can use viewport units & with good browser support

By setting an element's width, height and margins in viewport units, you can center it without using any other tricks.

Here, this rectangle has a height of 60vh and top and bottom margins of 20vh, which adds up to a 100vh (60 + 2*20) making it always centered, even on window resize.

#rectangle{
    width: 60vw;
    height: 60vh;
    margin: 20vh auto;
}

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