How to position overlay fixed div to the center with css?

Question

I have a div that appears once I have clicked some action to another element in the website. It is a fixed div that appears on top of all divs and

Answer 1

You can set top, right, bottom, left to 0 and margin to auto to vertically and horizontally center align fixed div. But this will only work if you set the height of div using CSS.

.overlay-box {
    background-color: #fff;
    border: 1px solid #000;
    text-align:center;
    height: 200px;/*height needs to be set*/
    width: 550px;
    margin: auto;
    position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
}

If you can't set the height then consider using flex layout https://css-tricks.com/snippets/css/a-guide-to-flexbox/