Final answer:
Arnold should use CSS to maintain a consistent visual style, provide a structured and accessible interface, and ensure website responsiveness on various devices.
Step-by-step explanation:
Arnold should use Cascading Style Sheets (CSS) when designing his web pages for several key purposes. Firstly, CSS allows Arnold to keep a consistent visual style across his website, which can help maintain the site's brand identity and improve user experience. This includes defining the fonts, colors, and backgrounds, as well as the overall layout of the pages. Secondly, CSS can help Arnold provide a structured and accessible user interface, which is important for all visitors, including those with disabilities. By using CSS, he can control spacing and the way media and other visuals are displayed, offering time and space for users to consume the content comfortably. Lastly, CSS is pivotal for responsive design, ensuring that the website is functional and visually appealing across different devices with varying screen sizes.