When taking a bird eyes view on CSS it will be hard to oversee that CSS is a mixture of different concepts that evolved over time with a lot of inconsistentsies. It is possible to make it work, but it's not pretty.

Same for HTML. If the web would be reimagined today, there is a very low chance that we would create HTML as is.

the biggest problem with html/css is that they are tightly coupled. you can't meaningfully modify a layout with css alone.

second biggest problem is "no stricter mode". so even wrong or useless html/css code goes unflagged and is treated as it is normal.

CSS is way too powerful.