Matching an empty input box using CSS

In modern browsers you can use :placeholder-shown to target the empty input (not to be confused with ::placeholder).

input:placeholder-shown {
    border: 1px solid red; /* Red border only if the input is empty */
}

More info and browser support: https://css-tricks.com/almanac/selectors/p/placeholder-shown/

If only the field is required you could go with input:valid

#foo-thing:valid + .msg { visibility: visible!important; }

 <input type="text" id="foo-thing" required="required">
 <span class="msg" style="visibility: hidden;">Yay not empty</span>

See live on jsFiddle

OR negate using #foo-thing:invalid (credit to @SamGoody)

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