PubNub provides a real-time solution for broadcasting CSS updates to websites or HTML5 mobile apps, allowing users to instantly change the appearance of web elements on a page. By utilizing a simple interface, users can modify CSS in a text box and broadcast these changes to all viewers of the page by clicking a "Broadcast CSS Changes" button. This functionality is enabled through PubNub's publish-subscribe model, where changes are sent over a defined channel and applied to the targeted element's style attributes in real-time. This method facilitates dynamic updates like altering design, images, or content without refreshing the page, and though the broadcast button can be hidden for privacy, users must sign up for a PubNub account to access these capabilities.