`margin:auto;` doesn't work on inline-block elements

Question

I have a "container" div to which I gave margin:auto;.

It worked fine as long as I gave it a specific width, but now

Answer 1

It is no longer centered because it now flows on the page in the same way inline elements do (very similarly to img elements). You will have to text-align: center the containing element to center the inline-block div.

#container {
    border: 1px solid black;
    display: inline-block;
    padding: 50px;
}
.MtopBig {
    margin: 75px auto;
    position: relative;
}
.center {
    text-align: center;
}