Class attribute set but not displayed in some cases

I am creating dynamically an HTML table in JavaScript, and when a condition is true, I set one class attribute to the tr element:

var tablaindice = result.tablaindice;

for (var key in tablaindice)
{
    var trautor = document.createElement("tr");


    var tdart = document.createElement("td");
    tdart.innerHTML = key;
    var tdcitas = document.createElement("td");
    tdcitas.innerHTML = tablaindice[key];
    if(key == result.indiceh)
    {
       trautor.setAttribute("class", "indiceh");
    }

    trautor.appendChild(tdart);
    trautor.appendChild(tdcitas);
    tbody.appendChild(trautor);
}

And this is the CSS code:

.indiceh{
    background-color: lightblue;
}

The class attribute is always set to the right element, but in some cases the blue background is displayed and in others it doesn't. Why is this happening?