[css] 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?

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/

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%));

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%);
}

<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;  
}