How can we preview a page using Salesforce lightning component?

Create a Lightning component app and click on preview to see the result.

For example:

<aura:application extends="force:slds">
<c:hello />
</aura:application>

<aura:component>
<div class="white">
Hello, HTML!
</div>
<h2>Check out the style in this list.</h2>
<ul>
<li class="red">I'm red.</li>
<li class="blue">I'm blue.</li>
<li class="green">I'm green.</li>
</ul>
</aura:component>

thanks