Web designers generally don’t increase the right-hand margin specifically to account for the scroll bar. Instead, modern Web Design primarily relies on responsive design principles and flexible layouts using CSS (Cascading Style Sheets) to ensure that web content is correctly displayed across different devices and screen sizes.

Most browsers automatically handle the presence of the scroll bar without interfering with the content layout, as they place the scroll bar adjacent to the content area. Responsive layouts typically use percentages, flexible grids, or media queries to adapt to various screen resolutions, which negate the need to manually alter margins for scroll bars.

However, in certain scenarios where precision layout is necessary, designers might use CSS properties such as box-sizing or overflow to better control how content fits within the viewport and manage scroll bar appearance. By utilizing tools like these, web designers ensure seamless and functional designs without the need for manual margin adjustments explicitly for scroll bars.