Please stop saying "CSS is for appearance, Javascript is for behavior."
Not since the 1990s has it been feasible to write a nontrivial app in which javascript doesn't impact the appearance of a web application.
The real choice is whether one does 50% of the "graphical stuff" in CSS and the other 50% in javascript... Or, instead, one just does 100% of the "graphical stuff" in javascript (independent of the merits of either approach- It may still make more sense to leverage CSS as much as possible, though I'm a skeptic of that viewpoint as well.)
Not since the 1990s has it been feasible to write a nontrivial app in which javascript doesn't impact the appearance of a web application.
The real choice is whether one does 50% of the "graphical stuff" in CSS and the other 50% in javascript... Or, instead, one just does 100% of the "graphical stuff" in javascript (independent of the merits of either approach- It may still make more sense to leverage CSS as much as possible, though I'm a skeptic of that viewpoint as well.)