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<!DOCTYPE html>
<style>
.outer {
/* Invert color/background so that it has more chances to disagree with the default background-color. */
color: Menu;
background-color: MenuText;
}
.inner {
height: 100px;
width: 100px;
padding: 10px;
}
</style>
<div class="outer">
<div class="inner">X</div>
</div>