Why CSS is called cascading

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  • July 19, 2024 at 2:58 pm #6453
    designboyo
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      CSS is called “cascading” because of the way styles are applied to HTML elements. The term “cascading” refers to the process by which styles are prioritized and applied in a specific order, considering multiple factors. Here’s a detailed explanation of why CSS is called cascading:

      1. Cascade Order

      The “cascade” in CSS refers to the order in which different styles are applied to an element. When multiple style rules apply to the same element, CSS uses a specific order to determine which rule takes precedence. This order is based on:

      1. Inline Styles: Styles written directly within an HTML element using the style attribute have the highest priority.
      2. Embedded Styles: Styles within a <style> tag in the HTML document head.
      3. External Styles: Styles linked via external CSS files using the <link> tag.
      4. Browser Default Styles: Styles that the browser applies by default.

      2. Specificity

      CSS uses specificity to determine which rules apply when multiple rules could be applied to the same element. Specificity is calculated based on the types of selectors used:

      • Inline styles have the highest specificity.
      • ID selectors (#id) have high specificity.
      • Class selectors (.class), attribute selectors ([type="text"]), and pseudo-classes (:hover) have medium specificity.
      • Element selectors (div, p, etc.) and pseudo-elements (::before, ::after) have the lowest specificity.

      3. Importance (!important)

      Using !important in a CSS rule overrides all other declarations, regardless of the cascade order and specificity. This is used to force a specific style to be applied.

      4. Source Order

      When specificity and importance are the same, the order of appearance in the CSS file determines which style is applied. Later styles override earlier ones if they have the same specificity and importance.

      Example: Understanding the Cascade

      Here’s an example that demonstrates the cascading nature of CSS:

      HTML:

      <!DOCTYPE html>
      <html lang=”en”>
      <head>
      <meta charset=”UTF-8″>
      <meta name=”viewport” content=”width=device-width, initial-scale=1.0″>
      <title>Cascading Example</title>
      <style>
      /* Embedded style */
      p {
      color: green;
      }
      </style>
      <link rel=”stylesheet” href=”styles.css”>
      </head>
      <body>
      <p id=”text” class=”highlight”>This is a paragraph.</p>
      <style>
      p {
      color: blue !important;
      }
      </style>
      </body>
      </html>

      CSS (styles.css):

      /* External style */
      p {
      color: red;
      }

      #text {
      color: purple;
      }

      .highlight {
      color: orange;
      }

      Explanation:

      1. Inline Style:
        • If the <p> element had an inline style, it would take the highest priority, e.g., <p style="color: black;">This is a paragraph.</p>. Since there’s no inline style here, we move on.
      2. Embedded Style:
        • The embedded style in the <style> tag at the top of the HTML document sets the color to green. But it’s overridden by the later embedded style with !important, which sets the color to blue.
      3. External Style:
        • The external CSS sets the color to red, but it’s overridden by the embedded styles and the more specific ID selector and class selector.
      4. ID and Class Specificity:
        • The external CSS also has an ID selector (#text) setting the color to purple and a class selector (.highlight) setting it to orange. The ID selector has higher specificity than the class selector, so it would normally take precedence if !important was not used.
      5. Importance:
        • The embedded style in the second <style> tag uses !important, which overrides all other declarations, making the text color blue.

      CSS is called “cascading” because of how it determines which styles to apply based on a combination of factors:

      • Cascade Order: Inline, embedded, external, and browser default.
      • Specificity: How targeted the selectors are.
      • Importance: Using !important to override other rules.
      • Source Order: Later rules override earlier ones if they have the same specificity and importance.
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