Hello I'm new to Salesforce and I'm doing the trailhead to learn.
But I'm stuck at this challenge in Lightning Module.

Here's the challenge:
Create a simple Lightning component with a styled headline. The headline must use an H1 tag with a CSS class that enforces a 24px font-size. Make the component available in the Navigation Menu of Salesforce1.The component must be named 'MyLightningComponent'.
The component must include an H1 tag with a CSS class named 'headline'. The 'headline' CSS class must set a font-size of 24px.
The Lightning Component tab that is added to Salesforce1 must be called 'MyLightning'.

And here's what I did:

MyLightningComponent.cmp

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

MyLightningComponentApp.app

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

MyLightningComponent.css

.THIS {
}

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

I created the tabs in the Salesforce1 and everithing looks right but I can't pass the challenge because it's aways return a error saying "The component does not include an H1 tag with a 'headline' CSS class"

I do no what to do with this error.