ref.html

<style>
span { color: green; background-color: green; }
</style>

<div><span>にほんご</span><br>日本語</div>

test.html

<style>
rt { color: green; background-color: green; }
</style>

<div></div><ruby><rb>日本語</rb><rt>にほんご</rt></ruby></div>