I just spent the past 4 hours making a breadcrumb layout in CSS. I hate making changes to fancy layout CSS. Does that mean the CSS language itself is technical debt? :P
(Yes, I know about and heavily abuse less/sass and associated libraries.)
Shadow DOM and WebComponents solve this better by encapsulating styling and markup concerned with the same widget or object. Without those you still have styling concerned with different markup in the same place. You can use less/sass but it's not true encapsulation - you still have to pay attention that you don't break styling for unrelated stuff when changing styling. Thus the "unwanted pain".
I've been loving React for the past 6 months, but yeah Shadow DOM and WebComponents have exactly the right idea WRT encapsulation.
My main CSS gripe is purely layout. I want to do relatively simple things like make responsive multi-column layouts using relative widths where one of the columns is fixed. Turns out that's hard and needs javascript. Flexbox is a huge step forward for making interesting layouts that were difficult in CSS and trivial in UI framework layout systems.