How to measure/determine the quality of the CSS code?

As some of the previous answers mention:

• Low specificity selectors
• Component-based styles

Basically use BEM (lowest possible specificity selectors and component-based styling) or something like Tailwind CSS (reusable classes without overriding styles everywhere).

If the css is modular, reusable and scalable.

How large the custom stylesheet is in terms of file-size.

Use of external css frameworks like Bootstrap or Foundation.

For large projects, if a preprocessor like Sass or Less is being implemented with properly organized functions, mixins, variables, etc.

• Usage of low/high specificity selectors.
• Styles with less noise
• Readable styles (is readable top to bottom)

Forgotten that !important still exists.