What is CSS?

Kristian Ole Rørbye

By: Kristian Ole Rørbye

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CSS (Cascading Style Sheets) is a fundamental technology used in web development to control the presentation and layout of web pages. It is a stylesheet language that describes how HTML elements should be displayed on screen, paper, or in other media. By separating content (HTML) from presentation (CSS), CSS allows for greater flexibility and control in the specification of web page characteristics.

The Role of CSS in Web Development

CSS plays a critical role in web development by enabling developers to define styles for web documents. These styles include layout designs, colors, fonts, spacing, and many other visual elements. The primary purpose of CSS is to enhance the user experience by making web pages more visually appealing and easier to navigate.

When a browser loads a web page, it reads the HTML to understand the content and the structure of the page. CSS is then used to style this HTML content. Without CSS, web pages would look very plain, with all elements displayed in a single column with no design or color differentiation.

How CSS Works

CSS works by associating rules with HTML elements. These rules are made up of selectors and declarations.

  • Selectors indicate which HTML elements the style should be applied to.
  • Declarations consist of properties and values that define how the selected elements should be styled.

For example, consider a simple CSS rule:

p {
    color: blue;
    font-size: 14px;
}

In this example, p is the selector, and it targets all <p> (paragraph) elements in the HTML. The declarations within the curly braces {} specify that the text color should be blue and the font size should be 14 pixels.

The Cascade in CSS

The “cascading” part of CSS refers to the way in which styles are applied and prioritized. When multiple style rules apply to the same element, CSS determines which rule takes precedence based on the source of the rule, the specificity of the selectors, and the order in which the rules are defined. This is known as the cascade.

CSS rules can come from three main sources:

  1. Author stylesheets: CSS rules defined by the developer in the stylesheet linked to the web page.
  2. User stylesheets: CSS rules set by the user in their browser settings, often used for accessibility purposes.
  3. Browser stylesheets: Default styles provided by the web browser to ensure content is readable even without a custom stylesheet.

The cascade follows a principle where the last rule defined has the highest priority unless specificity or importance is applied. Specificity is calculated based on the types of selectors used. For example, an ID selector (#id) is more specific than a class selector (.class), and therefore takes precedence if both are applied to the same element.

Types of CSS

CSS can be applied to HTML in three ways:

Inline CSS: Styles are applied directly within an HTML element using the style attribute. Inline styles have the highest priority over other types of CSS but are typically avoided due to poor maintainability.

<h1 style="color: red;">This is a heading</h1>

Internal CSS: Styles are placed within a <style> tag in the <head> section of an HTML document. Internal CSS is useful for applying styles to a single document.

<head>
    <style>
        h1 {
            color: blue;
        }
    </style>
</head>

External CSS: Styles are defined in an external .css file, which is linked to the HTML document using the <link> tag. External CSS is the most efficient and scalable way to apply styles across multiple pages of a website.

<link rel="stylesheet" href="styles.css">

CSS Syntax and Structure

A CSS rule-set consists of a selector and a declaration block:

  • Selector: The part of the CSS rule that selects the HTML element(s) to style.
  • Declaration block: A list of style declarations within curly braces {}.

Each declaration includes a property and a value, separated by a colon :. Multiple declarations are separated by semicolons ;.

Example:

h1 {
    color: green;
    text-align: center;
}

In this rule-set:

  • h1 is the selector.
  • color and text-align are properties.
  • green and center are values.

Advanced CSS Concepts

As web design evolves, so does CSS, with newer features enabling more complex and responsive designs. Some advanced CSS concepts include:

  • CSS Flexbox: A layout model that allows elements within a container to be distributed and aligned in various ways. It is particularly useful for designing complex layouts with minimal code.
  • CSS Grid: A powerful two-dimensional layout system that provides more control over rows and columns, making it easier to create complex web page layouts.
  • CSS Animations and Transitions: These are used to animate changes to CSS properties, adding dynamic effects to web pages.
  • CSS Variables: Also known as custom properties, these are entities defined by developers that contain specific values to be reused throughout a document.

Benefits of Using CSS

Using CSS has several advantages:

  • Separation of Content and Design: By keeping HTML for content and CSS for styling, developers can create cleaner, more manageable codebases.
  • Consistent Design: CSS allows for a uniform look across multiple pages of a website, ensuring consistency in design and user experience.
  • Responsive Design: CSS supports responsive design techniques, allowing web pages to adapt to different screen sizes and devices.
  • Faster Load Times: With CSS, web pages can load faster by reducing the amount of HTML needed and through the use of external stylesheets that can be cached by browsers.

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