An example of max-width and margin-auto

The first box bordered in blue is always 500 pixels no matter the width of the browser page. The second box bordered in green will be 500 pixels unless the browser window is too small so it will get smaller. Play with the size and see.

This is a box of 500 pixels wide that should be centered.

This is a box with max-width of 500 pixels that should be centered.

These two boxes are both 500 pixels width, with similar margins and centered but the second one has padding and thicker border.

This is a box that should be 500 pixels width with 50px top and bottom margins and centered.

This is a box that should be 500 pixels width with 50px top and bottom margins and centered. And padding of 50px all around and a border width of 20px.