Instruction: Define what CSS specificity is and give an example of how it works.
Context: This question is designed to test the candidate's understanding of CSS specificity rules and how they influence the application of styles.
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To give you a more concrete idea, let's consider an example involving three CSS selectors: an element selector, a class selector, and an ID selector. For instance, suppose we have the following CSS:
```css header { color: blue; }...