This is the component:

<aura:component implements='force:appHostable'>
    <h1 class="headline">My Lightning Component</h1>
</aura:component>

This is the app:

<aura:application >
    <h1>Hello Lightning App!</h1>
    <c:MyLightningComponent />
</aura:application>

This is the CSS:

.THIS{  
}

h1.THIS.headline {
    font-size:24px;
}

To me it seems to be doing what is it supposed to be doing. Which is "The component must include an H1 tag with a CSS class named 'headline'. The 'headline' CSS class must set a font-size of 24px."
Yet it complains that "The component is not using the correct CSS for the 'headline' class".
Anyone an idea of what I am doing wrong.
Much appreciated in advance.