Which Direction Do Selector Engines Read, Exactly?

"In which direction do selector engines read, exactly?"

The conventional wisdom holds that CSS selector engines commonly read from right to left. However, the precise direction of evaluation can vary depending on the browser and implementation.

Typically, browsers evaluate selectors from right to left to navigate combinators. For instance, in the selector section > div.second > div.third, the engine checks if an element is div.third before examining if its parent is div.second and further up the hierarchy.

Regarding the performance comparison between .name and div.name, the exact order of evaluation can fluctuate and vary broadly across browsers. Even theoretical performance largely relies on the implementation.

Despite the right-to-left evaluation across combinators, it's unlikely that this order extends down to simple selector levels within a compound selector. For example, in the selector div:hover[data-foo="bar"].name:nth-child(5)::after, there's no set guarantee that the criteria will be checked in a strict right-to-left manner.

Instead, certain types of simple selectors may be prioritized for performance reasons. For instance, ID selectors are generally handled first. So, whether a selector is written as div#foo.bar:first-child or div.bar:first-child#foo, Gecko (used by Firefox) will evaluate the ID and class first, regardless of their position.

Ultimately, it's difficult to predict which selector will perform faster in a given context. Performance can differ based on document structure, implementation, and even across different browsers.

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