> drops all resolved CSS rules on element, its siblings, its parent and all children.

Browsers optimize this, too - they try hard to keep up metadata structures that tell them whether they need to invalidate styling or not. (This is why mutating the stylesheet is so expensive - browsers have to freshly rebuild the supporting data structures. They could optimize that too, but it's a lot of effort for a rare case.)

So if there's no rules mentioning an [a] attribute in the page, el.setAttribute("a", ...) will generally not do anything.

(One of the benefits of shadow DOM is that the shadow trees are scoped away from the main tree and each other, so you can have more expensive rules in them without making the rest of the page slow. Yay for componentization!)