JSFiddle - React, Tailwind, and code Playground

HTML

<form action="fichier.php" name="form" >
 	<input type="number" min=0 name="appels" />
 	<input type="number" min=0 name="reponses" />
 	<input type="text" name="resultat" />
 </form>

CSS

.mauvais {background: #FF9847;}
.bon {background: #86DF44;}

JavaScript

function result(){

	var numApp = parseInt(app.value),
		numRep = parseInt(rep.value),
		resultat = (numRep / numApp) * 100;
	
    if (!isNaN(resultat) && resultat <= 100) {
        res.value = resultat;
        res.className = (resultat < 33) ? "mauvais" : "bon";
    }
    else {
        res.value = '';
        res.className = '';
    }
}

var app = document.form.appels,
	rep = document.form.reponses,
    res = document.form.resultat;

app.addEventListener("keyup", result, false);
rep.addEventListener("keyup", result, false);