How do you ensure your designs are responsive and accessible across different devices and platforms?

To ensure responsiveness, I use a mobile-first approach, designing for smaller screens and then scaling up. I also make use of flexible grids and layouts, and CSS media queries to adapt the design to various screen sizes. For accessibility, I follow WCAG guidelines, ensuring proper contrast, readable fonts, and keyboard navigation. Regular testing on different devices and assistive technologies is essential to guarantee accessibility and responsiveness.