JSFiddle - React, Tailwind, and code Playground

HTML

<p>test 
    <span class="underline">Je voudrai surligner une ligne en entier</span>
    <br>
    <span class="underline">&nbsp;</span>
</p>

CSS

p {line-height:16px}
.underline {background:yellow;display:block;text-align:center}