How Does Css Work Under The Hood

CSS Programming

When it comes to web development, CSS (Cascading Style Sheets) plays a crucial role in styling and designing web pages. It allows developers to control the layout, colors, fonts, and other visual aspects of a website. But have you ever wondered how CSS works under the hood? In this article, I’ll take you on a deep dive into the inner workings of CSS, providing insights and personal commentary along the way.

The Cascade

The first thing to understand about CSS is the concept of the cascade. The cascade refers to the order in which styles are applied to an HTML element. When multiple CSS rules target the same element and have conflicting styles, the cascade determines which style takes precedence.

Personally, I find the cascade to be both powerful and frustrating at times. It allows for flexibility and customization but can also lead to unexpected results if not managed correctly. Understanding how the cascade works is essential for mastering CSS.

Selector Matching

Before the styles can be applied, CSS needs to determine which elements on the page match each CSS rule. This process is called selector matching. Selectors are used to target specific elements based on their tag names, classes, IDs, or other attributes.

Selector matching can be a complex and time-consuming process, especially if you have a large number of CSS rules or complex selectors. The browser must traverse the DOM (Document Object Model) tree and compare each element to the selectors to determine if there is a match. This is where optimizing your CSS selectors can significantly improve performance.

Rendering and Layout

Once the CSS rules are matched to the elements, the browser proceeds with rendering and layout. Rendering involves calculating the size, position, and appearance of each element on the page, taking into account the styles defined in the CSS.

Layout, on the other hand, deals with arranging the elements in their correct positions based on their calculated dimensions. This process can be complex, especially when dealing with responsive designs that adapt to different screen sizes and orientations.

Specificity and Inheritance

Two important concepts in CSS are specificity and inheritance. Specificity determines which style rule takes precedence when there are conflicting styles. It is determined by the type of selector used, such as tag name, class, or ID, as well as any inline styles.

Inheritance, on the other hand, allows styles to be applied to child elements based on their parent elements. This can save a lot of time and effort when styling large websites, as you don’t need to apply the same styles to each individual element.

Browser Rendering Engine

Behind the scenes, CSS is processed by the browser’s rendering engine. Each browser has its own rendering engine, such as Blink for Google Chrome, Gecko for Firefox, and WebKit for Safari.

These rendering engines parse the CSS code, apply the styles to the elements, and render the final layout on the screen. Each rendering engine may have its own quirks and implementation details, which can lead to slight differences in how CSS is interpreted and rendered.


Understanding how CSS works under the hood is essential for becoming a proficient web developer. The cascade, selector matching, rendering and layout, specificity, and inheritance are all crucial concepts to grasp.

While CSS can be complex and sometimes frustrating, it offers immense power and flexibility in designing visually appealing websites. Embracing CSS and continually expanding your knowledge of its inner workings will undoubtedly make you a better web developer.