The :has() pseudo-class is a selector that selects elements based on the elements that it contains. According to the CSS Almanac, it is referred to as a "parent selector" because of its ability to select an element based on the child elements it contains. Use cases can be for stylizing similar elements and wanting to restrict that style to elements only containing a certain child element.
The empty-cells CSS property is for empty table cells and depicts whether or not the browser will show the border and background of an empty cell. The property acts like the visibility property but for table cells. A use case for this property is if you may not be certain a cell will contain data so instead of showing a blank table cell you can hide it in the case it is empty.
The cursor property is a super useful UX property that helps depict the functionality of links, buttons, or other elements of a page. That said, this one is restricted to browsers where a mouse is being used, so it doesn't affect mobile browsers.
The grid-template-areas property assigns keywords to a certain grid area. This means that if you want three grid areas labeled header, main, and footer, you'd assign those areas in the grid container and then the values to each respective container item (see code from link). This page is designed using grid-template-areas.
The z-index property gives a page the sense of a 3rd dimension. This property affects which elements overlap or get covered by others. However, if a parent element has a z-index of 1 and its child a z-index of 100, the child would only be overlapping its parent (and children below a z-index of 100), and not non-child elements with a z-index greater than 1. A common use case for the z-index property is for hiding nav menus or dropdown menus.
Scrollbar styling has evolved from an IE only property to a standardized one. However, even still the styling for Safari and Chrome requires the '-webkit' prefix. This is mainly used to add some flare to the scrollbar for a page or make it more personalized to the site rather than the generic scrollbar.