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    A CSS file (Cascading Style Sheets) is a plain text file with the .css extension that defines how HTML elements should be displayed on screen, paper, or other media. It separates content (HTML) from presentation (design and layout), allowing developers to control the look and feel of multiple web pages from a single file.

    Key purpose:

    • Control colors, fonts, spacing, and layout.

    • Apply responsive designs for different devices.

    • Enable reusability — one CSS file can style an entire website.

    Basic Structure of a CSS Rule:

    • Selector: Targets HTML elements (e.g., h1, .class, #id).

    • Declaration Block: Contains one or more property-value pairs inside { }.

    Example:

    h1 {
    color: forestgreen;
    font-weight: 700;
    }
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    This styles all <h1> elements with green text and bold weight.

    Using a CSS File in HTML: External CSS files are linked in the <head> section of an HTML document:

    <link rel="stylesheet" type="text/css" href="styles.css">
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    The href attribute points to the CSS file path. Once linked, all matching selectors in the CSS will style the HTML elements.

    What is a CSS page?
    What is CSS? Cascading Style Sheets (CSS) is a core language of the open web platform, and is used for adding style (e.g., fonts, colors, spacing) to Web documents. These pages contain information on how to learn and use CSS and on available software. They also contain news from the CSS working group.
    w3.org
    W3School
    https://www.w3schools.com/Css/css_intro.asp
    CSS Introduction - W3Schools
  1. Mozilla Developer
    https://developer.mozilla.org › ... › Core › Styling_basics …

    What is CSS? - Learn web development | MDN - MDN …

    Nov 13, 2025 · CSS is a language that lets you control how HTML elements look in the browser. Learn the basics of CSS syntax, how it works with HTML and the …

  2. W3School
    https://www.w3schools.com › Css › css_intro.asp

    CSS Introduction - W3Schools

    CSS is the language we use to style a Web page. Learn what CSS stands for, how it can control the layout of multiple web pages, and how to create and apply external stylesheets in .css files.

    Code sample

    body{
      background-color: lightblue;
    }
    h1{
      color: white;...
  3. FileInfo.com
    https://fileinfo.com › extension › css

    CSS File - What is a .css file and how do I open it?

    Aug 19, 2021 · What is a CSS file? A CSS file is a cascading style sheet (CSS) file …

    • Developer: W3C
    • Used by: Adobe Dreamweaver 2021
    • Popularity: 4.0 | 213 Votes
  4. Wikipedia
    https://en.wikipedia.org › wiki › CSS

    CSS - Wikipedia

    Cascading Style Sheets (CSS) is a style sheet language used for specifying the presentation and styling when it’s none of a document written in a markup …

  6. GeeksForGeeks
    https://www.geeksforgeeks.org › css › what-is-css

    What is CSS? - GeeksforGeeks

    Jul 23, 2025 · CSS, which stands for Cascading Style Sheets is a language in web development that enhances the presentation of HTML elements. By applying styles like color, layout, and spacing, CSS …

  7. File Format Docs
    https://docs.fileformat.com › web › css

    What is CSS (Cascading Style Sheets) File Formatand …

    CSS files are external stylesheets that describe how HTML elements are displayed on the screen, paper, etc. Learn about the brief history, syntax, and usage of CSS …

  8. Elementor
    https://elementor.com › blog › what-is-css-guide

    What is CSS? A Beginner's Guide to Cascading Style …

    Oct 28, 2025 · CSS stands for Cascading Style Sheets. It is the language used to style the visual presentation of a website. HTML provides the structure (the …

  9. World Wide Web Consortium (W3C)
    https://www.w3.org › Style › CSS

    Cascading Style Sheets - World Wide Web Consortium …

    32 rows · Apr 23, 2026 · Cascading Style Sheets (CSS) is a core language of the open web platform, and is used for adding style (e.g., fonts, colors, spacing) to …

  10. Hostinger
    https://www.hostinger.com › tutorials › what-is-css

    What is CSS: Cascading Style Sheet Explained for …

    Aug 21, 2025 · CSS lets you stylize a list of design rules in a separate file that are referenced in the HTML markup. This makes the actual HTML markup much …

  11. FreeCodecamp
    https://www.freecodecamp.org › learn › lecture-what-is-css › what...

    What Is CSS, and What Is Its Role on the Web? - freeCodeCamp.org

    CSS, which stands for Cascading Style Sheets, is a crucial component of modern web development. It's a markup language used to apply styles for HTML. In simpler terms, if HTML is the structure of a web …

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