How CSS Works: Complete Guide to CSS

Posted on | by Michael Parisi

1. The Cascade

CSS, by its very nature, stands for “Cascading Style Sheets,” and understanding the cascade is crucial to mastering CSS.

  • Understanding specificity and inheritance:
  • Specificity determines which CSS rule is applied by the browser. It’s calculated based on the number and type of selectors. For instance, an ID selector has higher specificity than a class selector.
  • Inheritance is the mechanism by which styles are passed from a parent element to its children. Some properties, like font-family, are naturally inherited, while others, like padding, are not.
  • Importance of source order:
  • If two selectors have the same specificity, the browser will use the rule that comes last in the CSS source. This makes source order essential when determining which styles are applied to an element.

2. The Box Model

Every element in a webpage can be thought of as a rectangular box, and understanding this box model is key to controlling layout and appearance.

  • Margins, borders, padding, and content:
  • Margin: The space outside the border, used to separate the element from its neighbors.
  • Border: Goes around the padding and content, defining the edge of the element.
  • Padding: The space between the content and the border.
  • Content: The actual information in the box, such as text, images, or other media.
  • Difference between box-sizing: content-box and box-sizing: border-box:
  • content-box (default): Width and height properties apply only to the content of the box, not the padding or border.
  • border-box: Width and height properties include content, padding, and border (but not margin). This makes designing responsive and fluid layouts simpler.

3. CSS Selectors

Selectors are the means by which we target and apply styles to HTML elements.

  • How they target elements: Selectors can target elements based on their type, class, ID, attributes, or even their relation to other elements.
  • Categories of selectors:
  • Type selectors: Target elements by their node name (e.g., h1, p).
  • Class selectors: Target elements by class attribute (e.g., .classname).
  • ID selectors: Target elements by their ID attribute (e.g., #idname).
  • Attribute selectors: Target elements based on their attributes and values (e.g., [attr=value]).
  • Pseudo-class and pseudo-element selectors: Target elements based on their state or position (e.g., :hover, ::before).

4. Applying CSS Styles

There are several ways to apply CSS styles to HTML documents, each with its advantages and use cases.

  • Inline styling: Directly within an HTML element using the “style” attribute. It’s the most specific way of applying styles but is often avoided due to its lack of reusability and potential for clutter.
  • Internal (embedded) styling: Within the <style> tag in the <head> section of an HTML document. It applies to that specific document only.
  • External CSS files: By linking to an external .css file using the <link> tag. This is the most common and recommended method, especially for larger websites, as it promotes reusability and separation of concerns.

5. Rulesets and Declarations

CSS styles are defined using rulesets, which are combinations of selectors and declarations.

  • Structure of a CSS rule: A rule is composed of a selector (or a group of selectors) followed by a declaration block. The declaration block contains properties and their respective values, which define the styling. For example:
  selector {
      property: value;
      another-property: another-value;
  }
  • Importance of property-value pairs: These pairs define the actual styles that get applied to elements. Each property specifies a particular aspect of styling (like color, font-size, or margin), and the value defines how that aspect should appear or behave.

With this understanding of how CSS works, you’ll be better equipped to design and troubleshoot webpages, ensuring they look and function as intended across various devices and browsers.

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