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Lesson 9: Core Selectors: ID Selectors (#)

**La Maîtrise de CSS : De zéro à expert en 100 leçons** *(Alternative, slightly punchier version using a common French idiomatic structure for full coverage: CSS : Maîtrisez l'essentiel en 100 leçons, but the first option is closer to the original progression.)*

9. Core Selectors: ID Selectors (`#`)

ID selectors target a single, unique element within the HTML document. They are identified by the hash symbol (#).

Defining and Using an ID

HTML: Add the id attribute to a single element. html

My Website
CSS: Target the ID by preceding the ID name with a hash (#). css #main-header { background-color: #333; color: white; padding: 20px; }

ID vs. Class: The Crucial Difference

Feature	Class Selector (`.`)	ID Selector (`#`)
Usage	Used many times on a page	Used exactly once per page
Purpose	Styling reusable components	Targeting highly specific, unique elements (often for JS/Accessibility)
Specificity	Low/Medium	Very High (Hard to override)

Modern CSS Best Practice:

In modern web development, developers tend to rely almost exclusively on Class selectors for styling. ID selectors are often reserved for JavaScript manipulation, linking (anchor tags), or accessibility features, rather than defining cosmetic styles.

Why Avoid IDs for Styling? Because their high specificity makes stylesheets hard to manage and brittle (inflexible).