Non-static elements will not inherit their parent's opacity in IE for no good reason. This can lead to unexpected behaviour when you want to fade/hide an element and all its children.

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To fix it, give the parent element defining the opacity a non-static positioning. For example:

.parent {
  opacity: 0.2;
  position: relative; /* for IE */
}

While the linked article describes this problem for IE9 and below, I have encountered the same issue in IE10 and IE11.

Just go away, Internet Explorer!