Help please - I am trying to complete the Lightning Challenge here:

https://developer.salesforce.com/trailhead/lightning_components/lightning_components_creating

But when I submit I am told it is wrong:

The component does not include an H1 tag with a 'headline' CSS class

My component reads:

<!-- MyLightningComponent.cmp -->
<aura:component implements="force:appHostable"> 
    <div class="headline">
        <H1>Hello Lightning Component!</H1>
    </div>    
</aura:component>

and I have a Style:

.THIS {
    background-color: yellow;
    padding-top: 10px;
}

H1.THIS {
    font-size:24px;
}

.THIS H1{
    font-weight: bold;
    padding: 10px;
}

.THIS.headline {
    padding-top: 20px;
    font-size:24px;
}

.THIS .red {
    background-color: red;
    padding: 10px;
}

.THIS .blue {
    background-color: blue;
    padding: 10px;
}

.THIS .green {
    background-color: green;
    padding: 10px;
}

Can anyone describe what I have done wrong?