What problem do CSS Cascade Layers solve?

WHAT THIS TESTS The interviewer wants to know if you understand the cascade as a multi-step algorithm, not just specificity math. Cascade layers were added precisely because specificity wars and source-order fragility make large stylesheets unpredictable.

A GOOD ANSWER COVERS Layers introduce an explicit priority dimension that is evaluated before specificity. You declare an order once, for example @layer reset, base, components, utilities. Any rule in a later-declared layer wins over any rule in an earlier layer, even if the earlier rule has higher specificity. Within a single layer, normal specificity and source order apply. Critically, unlayered styles have the highest priority of all, sitting above every named layer. This lets a team drop a noisy third-party stylesheet into an early layer and trust their own component styles in a later layer to win without resorting to !important.

COMMON WRONG ANSWERS Saying layers increase specificity is wrong; they sit above specificity in the algorithm. Claiming a higher-specificity selector in an earlier layer beats a lower-specificity one in a later layer is also wrong. Forgetting that unlayered rules outrank all layered rules trips many candidates.

LIKELY FOLLOW-UPS How do you order layers when files load asynchronously? Answer: declare the empty layer order statement first so order is fixed regardless of when each layer is populated. How do !important and layers interact? Important reverses layer order, so important rules in earlier layers win. Can you nest layers? Yes, with dotted names like components.card.

ONE CONCRETE EXAMPLE Suppose a vendor grid library defines .grid .cell with specificity 0,2,0, and you want your own .cell rule with specificity 0,1,0 to override it. Without layers you would need !important or a more specific selector. With @layer vendor, app declared in that order, you put the vendor import in the vendor layer and your .cell in the app layer; your lower-specificity rule wins simply because app comes later in the layer order.