Edit in JSFiddle

<p>Normal text</p>

<div class="parent">
    First Text.
    <div class="child">
        Second Text.
        <div class="second-child">
            Third Text
            <div class="third-child">
                Forth Text
            </div>
        </div>
    </div>
</div>

html{
    font: 1em/1.5 "Helvetica Neue", Arial, sans-serif;
}
.parent{
    font-size: 1.2em;
}
.child{
    font-size: 1.4em;
}
.second-child{
    font-size: 1em;
}
.third-child{
    font-size: 1rem;
}