Is "margin: 0 auto" default for CSS divs?

I just took over a new project. The guy before me coded a lot o开发者_高级运维f:

div#headertop {
    margin: 0 auto; 
}

Is this really needed or is this the default anyway?

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No, it is not default, unless he also set

div {
    margin: 0 auto;
}

This is needed to center divs on the page, provided there is also a width set. The top is given no margin and the left and right are set automatically in relation to the page/containing element (based on the width of the div)

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All current browsers have margin: 0 as the default margin for div elements.

margin: 0 auto is not the same thing -- if the div has a fixed width, margin: 0 auto creates a div that is horizontally centered in its container.

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margin: 0 auto; is a common practice for horizontally centering a particular element.

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The default for a div is to be 100% wide, so centering it horizontally as a default does not make any sense.