Most of these suggestions don't concern the design, but rather the overall CSS code organization.
Some notes:
if you're using a preprocessor - or in general a build system - let an autoprefixer do the job of inserting vendor-prefixed variants for you: just set up the target browsers, and the autoprefixer does the job, and only when necessary (e.g.: it won't add them for border-radius anymore);
keep in mind that shorthand properties are double edged swords - meaning that they could override declarations that you don't want to be overridden: background and font are especially tricky for that;
comment your code, but remember that comments should explain why, not what;
it's notable that you mentioned responsive design without talking about media (or container!) queries: it's true that you can achieve responsiveness without media queries, but it's kind of limiting - also because device sizes aren't everything when it comes to responsiveness;
the usage of CSS frameworks is very debatable: if you've been given a design system that's not based on any of them, you'll end up adjusting the one you chose to your needs, de facto cancelling its benefits; also remind that utility classes are a violation of the separation of concerns principle;
there are several way to "test" your CSS - I think you should be more specific here; also, I'll add visual regression tests to your arsenal;
and the example for point 10 is the same of point 6.
Anyway, I'm glad that someone is still pushing to adopt best practices for CSS. All around, it seems that everybody thinks that Tailwind is the best thing after the invention of sliced bread.
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Most of these suggestions don't concern the design, but rather the overall CSS code organization.
Some notes:
border-radiusanymore);backgroundandfontare especially tricky for that;Anyway, I'm glad that someone is still pushing to adopt best practices for CSS. All around, it seems that everybody thinks that Tailwind is the best thing after the invention of sliced bread.