The optgroup HTML element is used to group related options within a select dropdown list, enhancing usability by organizing long lists of choices into logical categories. This improves navigation and selection efficiency for users.

The ol HTML element is used to create an ordered list, where items are presented in a specific sequence, typically numbered or lettered. This is ideal for instructions, rankings, or any content where the order is significant.

The object HTML element is used to embed external resources, such as images, videos, or interactive content. It can also serve as a container for plugins or other applications, offering flexibility in integrating diverse media types.

The noscript HTML element is used to provide alternative content for users whose browsers do not support JavaScript or have scripting disabled. It ensures that essential information or functionality is accessible even without scripts.

The nav HTML element is used to define a set of navigation links, typically for a website's main navigation menu. It helps organize and structure the navigation of a webpage, improving accessibility and user experience.

The meter HTML element is used to represent a scalar measurement within a known range or a fractional value, such as disk usage or voting percentages. It visually displays this data as a gauge, with attributes to define the range and current value.