I agree that there should be an "escape hatch" to implement stuff that evaded the great wisdom of the commitees. The Houdini project may become such: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Houdini

But I do also see the benefit of a declarative system for most cases. Adaptive layouts are hard to do with imperative code, and very easily lead to e.g. infinite loops, so it's nice to work with a declarative system that guarantees a stable layout. That said, CSS is far from an ideal such system with so much ad-hoc and legacy baggage.