Center fixed div with dynamic width (CSS)

I have a div that will have this CSS:

#some_kind_of_popup
{
    position: fixed;
    top: 100px;
    min-height: 300px;
    width: 90%;
    max-width: 900px;
}

Now, how can i make this div centered? I can use margin-left: -450px; left: 50%; but this will only work when the screen is > 900 pixels. After that (when the window is < 900 pixels), it will no longer be centered.

I can of course do this with some kind of js, but is there a "more correct" of doing this with CSS?

You can center a fixed or absolute positioned element setting right and left to 0, and then margin-left & margin-right to auto as if you were centering a static positioned element.

#example {
    position: fixed;
    /* center the element */
    right: 0;
    left: 0;
    margin-right: auto;
    margin-left: auto;
    /* give it dimensions */
    min-height: 10em;
    width: 90%;
}

See this example working on this fiddle.

Here's another method if you can safely use CSS3's transform property:

.fixed-horizontal-center
{
	position: fixed;
	top: 100px; /* or whatever top you need */
	left: 50%;
	width: auto;
	-webkit-transform: translateX(-50%);
	-moz-transform: translateX(-50%);
	-ms-transform: translateX(-50%);
	-o-transform: translateX(-50%);
	transform: translateX(-50%);
}

...or if you want both horizontal AND vertical centering:

.fixed-center
{
	position: fixed;
	top: 50%;
	left: 50%;
	width: auto;
	height: auto;
	-webkit-transform: translate(-50%,-50%);
    -moz-transform: translate(-50%,-50%);
    -ms-transform: translate(-50%,-50%);
    -o-transform: translate(-50%,-50%);
    transform: translate(-50%,-50%);
}

This works regardless of the size of its contents

.centered {
      position: fixed;
      top: 50%;
      left: 50%;
      transform: translate(-50%, -50%);
    }

source: https://css-tricks.com/quick-css-trick-how-to-center-an-object-exactly-in-the-center/

<div id="container">
    <div id="some_kind_of_popup">
        center me
    </div>
</div>

You'd need to wrap it in a container. here's the css

#container{
    position: fixed;
    top: 100px;
    width: 100%;
    text-align: center;
}
#some_kind_of_popup{
    display:inline-block;
    width: 90%;
    max-width: 900px;  
    min-height: 300px;  
}

This approach will not limit element's width when using margins in flexbox

top: 0; left: 0;
transform: translate(calc(50vw - 50%));

Also for centering it vertically

top: 0; left: 0;
transform: translate(calc(50vw - 50%), calc(50vh - 50%));