QLabel paints improper background when using HTML content

// Main Window:

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
    resize(400, 300);
    GradientWidget *widget = new GradientWidget;
    setCentralWidget(widget);
    QVBoxLayout *layout = new QVBoxLayout(widget);
    layout->addWidget(new QLabel("Label with a proper (transparent) background"));
    layout->addWidget(new QLabel("<b>HTML</b> label with an <i>improper</i> (inherited from parent) background"));
}

// Gradient Widget:

class GradientWidget : public QWidget {
protected:
    void paintEvent(QPaintEvent *event)
    {
        QLinearGradient gradient(event->rect().topLeft(), event->rect().bottomRight());
        gradient.setColorAt(0, Qt::white);
        gradient.setColorAt(1, Qt::darkYellow);
        QPainter painter(this);
        painter.fillRect(event->rect(), gradient);
    }
};