<style>
.notice {
display: inline-block;
border-bottom: solid 1px black;text-decoration: position: relative;
}

.notice-text {
font-size: 80%;
position: absolute;
display: block;
}
</style>

かきくけこ<span class="notice">あいうえお<span class="notice-text">いろは</span></span>さしすせそ

とか？

<span class="notice" title="いろは">あいうえお</span>
みたいなこともできるけど