Summary  
ID selectors in CSS enable targeting a single, unique HTML element by referencing its id attribute with a “#” prefix, ensuring styles apply exclusively to that element.

General domain of usage  
Web page styling

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An **ID selector** in CSS allows you to target a single, unique element on your page. To use an ID selector, add an `id` attribute to an HTML element, then reference that ID in your CSS by prefixing it with a hash (`#`). For example, if an element has `id="main-title"`, you select it in CSS with `#main-title`.

Each ID value must be unique within a page—no two elements should share the same ID. This uniqueness helps ensure that the styles you apply with an ID selector affect only the intended element. ID selectors are commonly used for elements that need a specific, one-off style or for elements you want to reference directly in scripts or navigation.

Which of the following is the correct syntax for selecting an element with the ID "header" in CSS?

Build a solid foundation in CSS and understand how styles shape the appearance of every webpage. Write and apply rules, target elements with selectors, and master the cascade and box model to create clean, organized designs. Gain the essential skills every web developer needs to style web content with precision and confidence.

Understand how CSS connects to HTML and defines the look of a webpage. Write clean, structured rules, manage multiple properties, and apply styles using external, internal, and inline methods for flexible control over your designs.

Target specific HTML elements with precision using selectors and combinators. Explore how the cascade, specificity, and inheritance determine which styles take effect when multiple rules overlap.

Apply visual styling that brings structure and balance to web layouts. Work confidently with colors, units, and the box model to control spacing, borders, and overall composition.

ID Selectors