[DevExpress Support Team: CLONED FROM T711309: ResponsiveBox - React demo does not work]
ResponsiveBox doesn't render HTML content if it's defined as follows:
JavaScript<Item>
....
<div className={'header item'}>
<p>Header</p>
</div>
</Item>
See the attached example.
As a workaround, you can use the render and component options as described in the following help sections to overcome this issue:
Markup Customization - Component
Markup Customization - Rendering Function