[html] Align a div to center

One of my websites involves a div whose size is variable and you won't know it ahead of time. it is an outer div with 2 nested divs, the outer div is the same width as the first nested div, which is the content, and the second nested div right below the content is the caption, which must be centered. Because the width is not known, I use jQuery to adjust accordingly.

so my html is this

<div id='outer-container'>
<div id='inner-container'></div>
<div id='captions'></div>
</div>

and then I center the captions in jQuery like this

captionWidth=$("#captions").css("width");
outerWidth=$("#outer-container").css("width");
marginIndent=(outerWidth-captionWidth)/2;
$("#captions").css("margin","0px "+marginIndent+"px");

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