An HTML table designed with CSS can have a sleek and modern appearance, with visually appealing formatting and layout. Using CSS, the table's rows and columns can be given specific widths and heights, and the cells can be aligned and spaced evenly. The table's headers can be distinguished from the rest of the cells through font and background color changes. Borders and lines can also be added to the table for a cleaner and more defined look. Additionally, hover effects can be applied to specific rows or cells to create an interactive experience for the user. Overall, using CSS to design an HTML table allows for more customization and control over the table's appearance, making it more visually appealing and user-friendly.