CSS & CSSOM
Cascade, specificity, inheritance and computed style as a resolution algorithm you can reason about, rather than a fight you win with `!important`.
Stylesheet bytes become an indexed object model before they become style: how rules are stored for fast lookup, why CSS blocks rendering, and what each runtime way of changing style actually costs.
Origin and importance, then context, then element-attached styles, then layers, then specificity, then order — a six-step sort that decides every property on every element, in that order.
A three-part tuple compared left to right, not a score. What counts, what deliberately counts as zero, and how `:where()` and `:is()` turn specificity into something you choose.
From declared value to cascaded, specified, computed, used and actual value — which properties inherit, what `inherit`, `initial`, `unset` and `revert` each mean, and why reading style back can force layout.
Inherited properties holding a token stream, substituted at computed-value time — which makes them dynamic, scopeable per element, and capable of invalidating a very large subtree at once.
How matching actually works, why selector performance is almost never the bottleneck in a modern engine, and what the real cost is: how many elements a change forces the browser to restyle.