I dunno what else to call it, but you see it on millions of websites - including this demo! You can optionally declare a height, and use flexbox to have everything magically vertically aligned if that's your jam.
The advantage to using flexbox over transform for a modal dialog is that flexbox works with less code, is less taxing on the browser, and will not result in blury edges.
Dialog Title
The advantage to using flexbox over transform for a modal dialog is that flexbox works with less code, is less taxing on the browser, and will not result in blury edges.