How can I reorder my divs using only CSS?

Question

Given a template where the HTML cannot be modified because of other requirements, how is it possible to display (rearrange) a div above another div when they are not in that order in the HTML? Both divs contain data that varies in height and width.

<div id="wrapper">
    <div id="firstDiv">
        Content to be below in this situation
    </div>
    <div id="secondDiv">
        Content to be above in this situation
    </div>
</div>
Other elements

Hopefully it is obvious that the desired result is:

Content to be above in this situation
Content to be below in this situation
Other elements

When the dimensions are fixed it easy to position them where needed, but I need some ideas for when the content is variable. For the sake of this scenario, please just consider the width to be 100% on both.

I am specifically looking for a CSS-only solution (and it will probably have to be met with other solutions if that doesn't pan out).

There are other elements following this. A good suggestion was mentioned given the limited scenario I demonstrated—given that it might be the best answer, but I am looking to also make sure elements following this aren't impacted.

Answer 1

This solution uses only CSS and works with variable content

#wrapper   { display: table; }
#firstDiv  { display: table-footer-group; }
#secondDiv { display: table-header-group; }