JSFiddle - React, Tailwind, and code Playground

HTML

<span id="1">Ce texte devrait être en rouge !</span>
<input type="button" value="Appliquer les style en JavaScript" id="setStyle" />

CSS

#1{
    font-weight: bold;
    color: red;
}

JavaScript

function setStyle(){
    document.getElementById('1').style.fontWeight = 'bold';
    document.querySelector('#1').style.color = 'red';
}
document.getElementById('setStyle').onclick = setStyle;