You need to use text-align: center in the container element. Also, avoid using inline styles ...
<div style="margin: 0 auto; width: 656px; text-align: center;">
Demo
Note. Make sure you use text-align: left; if you have text inside the div otherwise they will also be centered.
As you commented, it looks like a center to me

However, you do not believe that they are perfectly concentrated, I would adjust the width containers and show you that they are centered ....


Using 2 buttons


Note. Make sure you also use CSS Reset ... So you get consistent styles across all browsers, although this should be centered regardless of the reset styles.
Mr. Alien
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