div.outer div.inner {
    display: none;
}
div.outer:hover div.inner {
    display: block;
}

div.first + div.second {
    display: none;
}
div.first:hover + div.second {
    display: block;
}

<input type="text" required oninvalid="this.setCustomValidity('Wow!')">

<input type="text" required oninvalid="this.setCustomValidity('Wow')" oninput="setCustomValidity('')" />