What does the principle of 'robust' imply in web design?

The principle of 'robust' in web design emphasizes the importance of ensuring that content remains accessible and functional across a wide range of user agents, including different browsers, assistive technologies, and devices. This principle recognizes that users may access web content using various platforms and tools, and thus, the design should accommodate these variations without sacrificing usability.

By adhering to robust design principles, developers create websites that are more resilient to changes in technology and user habits. This means that as new browsers or devices emerge, or as assistive technologies evolve, the web content will still be effectively usable and accessible. This ultimately leads to a better user experience for everyone, including people with disabilities who rely on specific tools to interact with web content.

The other options do not align with the core aspect of 'robust' in web design. Visual appeal, functionality limited to modern devices, or a focus solely on HTML do not encapsulate the broader commitment to ensuring that web content is usable for all users across various contexts.