Cascade Layer is the `@layer` rule in modern CSS. You declare named bands at the top of the stylesheet — `@layer reset, tokens, base, components, utilities, overrides;` — and any rule placed inside a layer takes its priority from the band, not from selector specificity or source order. A low-specificity rule in `utilities` wins over a high-specificity rule in `base`. The cascade becomes something you architect rather than something you fight.
The reason this matters in a WordPress refactor isn't the spec. It's that themes accumulate specificity wars. Block-library CSS lands at one weight, the parent theme overrides at another, plugin CSS pushes a third, and someone — usually two years in — starts reaching for `!important` to settle it. Once `!important` enters the codebase, every future change costs more.
When I refactored the thisismyurl.com stylesheet through what I call the Pass-5 iteration, moving the existing rules into named layers removed roughly 40 lines and 11 KB without changing a single visual outcome — the rules that had been written to beat each other could now be written once and trusted to land where intended. The byte savings are a side effect; the real win is that the next developer touching the theme can read the layer declaration and know, before reading a single selector, where their new rule needs to live.
Cascade Layer is worth adopting when your theme has accumulated three or more `!important` declarations, or when plugin CSS is fighting your theme. If your stylesheet is small and clean already, the refactor cost isn't justified.