Declarations not set in the highest priority source are passed on to a source of lower priority, such as the user agent style. One of the goals of CSS is to allow users greater control over presentation.
Someone who finds red italic headings difficult to read may apply a different style sheet.
One example of a widely used pseudo-class is Values may be keywords, such as "center" or "inherit", or numerical values, such as 200px (200 pixels), 50vw (50 percent of the viewport width) or 80% (80 percent of the window width). Before CSS, nearly all presentational attributes of HTML documents were contained within the HTML markup.
All font colors, background styles, element alignments, borders and sizes had to be explicitly described, often repeatedly, within the HTML.
This made documents more complex, larger, and more error-prone and difficult to maintain.