Why does CSS not support parent selectors?

Browsers parse HTML documents from front to back, inside and out. So it’s not uncommon to see the page load with the header and then the body content.

However, if CSS supports parent selectors, then all child elements of the page must be loaded before the HTML document can be rendered, because “parent selectors” means that the descendant elements affect the ancestor elements. If the descendant elements have not been loaded, how do they affect the style of the ancestor elements? As a result, web pages render much slower and the browser displays a whiteboard for a long time. You can render as much HTML as you load, which is especially necessary when the network speed is not very fast.

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