To dynamically position overlay elements such as modals or popups relative to their parent container, you can leverage CSS for layout flexibility and JavaScript for responsive behavior. First, ensure your parent container is styled with position: relative; so that all absolutely positioned children (like your overlays) can position themselves in relation to this parent. To make the overlays responsive to the container’s size, you can set their dimensions using percentages (e.g., width: 80%; height: auto;) or use viewport units (like vw and vh). Additionally, utilizing CSS Flexbox or Grid can help maintain a consistent layout, even as the container resizes.

For dynamic calculations, consider using JavaScript to listen for resize events and update the dimensions of your overlays accordingly. You can use the getBoundingClientRect() method to fetch the current dimensions of the parent container and then apply these values to your overlays, ensuring they do not overflow. For example, you might write a function that adjusts the overlay’s size and position based on the parent’s dimensions each time a resize event occurs. This combination of CSS for layout and JavaScript for dynamic adjustments will help ensure your overlays maintain a cohesive appearance relative to their parent containers.