Do CSS Selector Engines Read from Left to Right or Right to Left?

Do CSS Selector Engines Read Left to Right or Right to Left?

It is a common assumption that selector engines, which identify elements in an HTML document based on their class or ID, read from left to right. However, recent discussions suggest otherwise.

Right-to-Left Reading by Most Selector Engines

Evidence indicates that the majority of CSS selector engines adopt a right-to-left reading approach. This means they start evaluating the selector from the element furthest to the right and work towards the left.

Understanding the Right-to-Left Approach

While it may seem counterintuitive, reading right-to-left provides several advantages for selector engines:

• Improved performance: By checking the most specific part of the selector first (usually the element name or ID), engines can quickly narrow down the search.
• Efficient combinator handling: The right-to-left approach allows efficient handling of combinators (e.g., ">", " "), which connect elements.
• Optimized for common selectors: Many common selectors follow a right-to-left pattern, such as retrieving elements by class or ID.

Speed Considerations

Unfortunately, there is no definitive answer to whether div.name is faster than .name, as performance can vary depending on factors such as browser implementation and document structure.

Simple Selector Evaluation

While selector engines read right-to-left across combinators, it is unlikely that they do so within individual simple selector sequences (e.g., class, attribute, pseudo-class). This means that elements are typically checked for simple selectors in a left-to-right order, regardless of their position in the sequence.

Vendor Differences

It's important to note that optimizations and strategies for selector engine implementations can vary between browsers (e.g., Gecko and WebKit). For example, Gecko may prioritize checking for IDs first, even if it is not the furthest right in the selector.

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