7 CSS Interview Questions Every Developer Should Know

Despite the ever-evolving nature of web development, some technologies have firmly established themselves as long-standing pillars. CSS — or Cascading Style Sheets —  is one such technology that, since its inception in the late ’90s, continues to play a fundamental role in how we design websites today. CSS is the magic wand that transforms the fundamental structure of a webpage, built by HTML, into a visually compelling and user-friendly interface.

The technology’s widespread use – 97.1% of all websites use CSS – underscores its pervasive influence in shaping the internet’s look and feel. This ubiquity of CSS means there’s a constant demand for skilled CSS professionals who can leverage its potential to create engaging, responsive, and interactive web experiences.

However, with high demand comes high expectations. Employers are actively seeking developers who can use CSS to solve complex design problems, implement seamless user experiences, and efficiently manage styles across various device screens. Mastery over CSS, therefore, is a highly sought-after skill, essential for developers seeking to showcase their front-end prowess. And it’s a key consideration for recruiters aiming to bring in top-notch talent.

This post aims to guide you through a series of carefully selected and progressively more challenging CSS interview questions. We’ll provide comprehensive explanations and illustrative code snippets. Whether you’re a developer wanting to brush up your CSS skills or a hiring manager looking for the right questions to assess your candidates, this guide is designed to help you approach your next CSS interview with confidence.

Understanding CSS

When you land on a beautifully designed web page with dynamic visuals, attractive color schemes, and easy-to-navigate layouts, you’re witnessing the result of well-implemented CSS. CSS is a cornerstone of web development, and its main purpose is to describe how HTML elements should be displayed on the screen. 

CSS, along with HTML and JavaScript, forms the trinity of front-end web development. While HTML provides the structural skeleton of a webpage and JavaScript adds functionality, CSS is responsible for the aesthetics. It controls the layout of multiple web pages simultaneously, adjusts elements to different screen sizes, and applies consistent styling across a website. 

This language is stylesheet-based, which means you write rules that tell browsers how to render the HTML elements on a page. For example, you might use CSS to specify that all the heading elements on a website should be bold and blue, or that a specific paragraph should be indented and have a larger font size. 

CSS is not just about making websites look good; it also enhances the user experience. With it, developers can create responsive designs that adapt to different devices, improve load times by optimizing styles, and increase accessibility for users with special needs. 

But why does this all matter in an interview context? When technical teams are recruiting for roles that involve any aspect of front-end development, CSS knowledge is almost invariably a requirement. This is because understanding CSS is key to being able to create web applications that not only function well but also provide an excellent user experience. So, whether you’re a software engineer, a web developer, a UI/UX designer, or in any role that touches on the user interface, CSS should be in your wheelhouse.

The CSS Interview: What to Expect

A CSS interview is an opportunity for candidates to showcase their skills in transforming plain, static HTML into dynamic, visually appealing web interfaces. At the same time, it allows recruiters to assess whether a candidate can effectively use CSS to meet design specifications, troubleshoot layout issues, and enhance the user experience.

Interviews focusing on CSS will typically go beyond basic syntax and selectors, delving into advanced topics such as layout techniques (like CSS Grid and Flexbox), CSS preprocessors, animations, responsive design, performance optimization, and handling browser compatibility issues.

The types of questions asked can vary widely depending on the role and the company. For instance, a front-end developer position might include more in-depth questions about CSS animations and transitions, while a full-stack developer role could cover how CSS fits into the broader context of a project, including interactions with JavaScript and back-end technologies.

The roles that often require CSS skills are vast and varied. Besides the obvious front-end web developer and full-stack developer, other roles like UI/UX designers, software engineers, and even roles in marketing or SEO could require a firm understanding of CSS. In some interviews, you might be asked to write CSS code in real time or refactor an existing piece of CSS code. In others, you may need to review a snippet of CSS and HTML code and discuss how it could be improved for better efficiency and maintainability.

1. Implementing a Class for CSS Colors

This question examines a developer’s understanding of CSS colors, which play a crucial role in styling web pages, and their ability to model concepts using object-oriented programming.

Task: Write a JavaScript class called `CSSColor` that represents a color in CSS. This class should have a constructor that takes three arguments: red, green, and blue. It should also have a method called `toCSS()` that returns the color in CSS format.

Input Format: The constructor will take three integers, each representing the red, green, and blue components of the color. The `toCSS()` method will take no arguments.

Constraints:

• The values for red, green, and blue will be integers.
• Each value will be between 0 and 255, inclusive.

Output Format: The `toCSS()` method will return a string representing the color in CSS format.

Sample Input:

let color = new CSSColor(255, 0, 0);

Sample Output:

console.log(color.toCSS()); // "rgb(255, 0, 0)"

Sample Code:

class CSSColor {

    constructor(red, green, blue) {

        this.red = red;

        this.green = green;

        this.blue = blue;

    }

    

    toCSS() {

        return `rgb(${this.red}, ${this.green}, ${this.blue})`;

    }

}

Explanation

The `CSSColor` class has a constructor that sets the red, green, and blue properties of the class based on the arguments passed in. The `toCSS()` method then returns a string that formats these properties in the CSS rgb format.

This question challenges developers to demonstrate their understanding of CSS colors and their capacity to use object-oriented programming to represent real-world concepts. This kind of problem-solving ability is invaluable in a professional setting, where developers must often create custom abstractions to solve unique challenges.

2. Implementing a Function to Apply CSS Styles

This question delves deeper into a developer’s knowledge of how CSS styles are applied to HTML elements through JavaScript. This checks their capability to manipulate the Document Object Model (DOM), an essential skill for building interactive web applications.

Task: Write a JavaScript function called `applyStyles` that takes a CSS selector and a style object as inputs and applies the styles to all elements that match the selector.

Input Format: The function will take two arguments: a string representing a CSS selector and an object where the keys are CSS properties and the values are the desired styles.

Constraints:

• The CSS selector will be a valid string selector.
• The style object will contain at least one property-value pair.

Sample Input:

applyStyles('p', { color: 'red', fontWeight: 'bold' });

Sample Code:

function applyStyles(selector, styles) {

    let elements = document.querySelectorAll(selector);

    for (let i = 0; i < elements.length; i++) {

        for (let style in styles) {

            elements[i].style[style] = styles[style];

        }

    }

}

Explanation:

The `applyStyles` function uses the `querySelectorAll` method to get all the elements that match the provided selector. It then iterates over these elements. For each element, it loops through each property in the styles object and assigns the corresponding value to that property on the element’s `style` object.

This question ups the difficulty from the previous one by not only requiring the candidate to work with CSS but also to manipulate HTML elements using JavaScript. It provides a good gauge of the candidate’s proficiency with JavaScript and their understanding of how CSS and JavaScript can interact in a web development context.

3. Implementing a Function to Rotate an Element

This question tests a developer’s expertise in transforming HTML elements using CSS, a powerful feature that allows developers to animate elements and create engaging user interfaces. 

Task: Write a JavaScript function called `rotateElement` that takes an HTML element’s id and a rotation angle as inputs and rotates the element to the specified angle.

Input Format: The function will take two arguments: a string representing the id of an HTML element and a number representing the rotation angle in degrees.

Constraints:

• The id will correspond to an existing HTML element.
• The rotation angle will be a valid number.

Sample Input:

rotateElement('myDiv', 45);

Sample Code:

function rotateElement(id, angle) {

    let element = document.getElementById(id);

    element.style.transform = `rotate(${angle}deg)`;

}

Explanation:

The `rotateElement` function uses the `getElementById` method to find the HTML element with the specified id. It then applies a rotation transformation to this element by setting its `transform` style to `rotate(${angle}deg)`, where `${angle}` is replaced with the provided angle.

This question requires an understanding of CSS transformations, which are a complex but powerful feature of CSS. Knowing how to use these transformations is crucial for creating modern, dynamic web pages. It also continues to test the candidate’s proficiency with JavaScript, particularly their ability to manipulate HTML elements and CSS styles.

4. Implementing a Function to Create a Grid Layout

This question delves into the developer’s knowledge of CSS grid layout, an advanced and powerful tool for creating responsive web layouts. It also tests their ability to generate HTML elements dynamically with JavaScript.

Task: Write a JavaScript function called `createGrid` that takes two arguments: the number of rows and the number of columns. The function should create a grid of `div` elements with the specified number of rows and columns and apply CSS grid layout to arrange these divs into a grid.

Constraints: Both the number of rows and columns will be positive integers.

Sample Input:

let grid = createGrid(3, 3);

Sample Code:

function createGrid(rows, columns) {

    let grid = document.createElement('div');

    grid.style.display = 'grid';

    grid.style.gridTemplateRows = `repeat(${rows}, 1fr)`;

    grid.style.gridTemplateColumns = `repeat(${columns}, 1fr)`;

    

    for (let i = 0; i < rows * columns; i++) {

        let cell = document.createElement('div');

        cell.textContent = `Cell ${i + 1}`;

        grid.appendChild(cell);

    }

    

    return grid;

}

Explanation:

The `createGrid` function begins by creating a new div element and setting its display style to ‘grid’. It then uses the CSS `gridTemplateRows` and `gridTemplateColumns` properties to define the grid’s structure, using the `repeat` function to create the specified number of rows and columns.

The function then enters a loop that runs once for each cell in the grid. In each iteration, it creates a new div, sets its text content to indicate its position, and appends it to the grid.

Finally, the function returns the grid element, which now has the desired grid structure and contains the appropriate number of cells.

This question raises the difficulty by requiring candidates to generate HTML elements dynamically and style them with an advanced CSS feature: the grid layout. This represents a practical task that developers might often face when creating complex, responsive web layouts.

5. Implementing a Function for Responsive Design

In this question, we’re testing the developer’s knowledge of CSS media queries, an important tool for creating responsive designs that adapt to different screen sizes. 

Task: Write a JavaScript function named `createResponsiveDiv` that creates a `div` element. This `div` should be styled such that it is 100% of the browser window’s width when the window is less than 600px wide and 50% of the browser window’s width otherwise.

Input Format: The function takes no arguments.

Constraints: The browser window’s width will be a positive number.

Output Format: The function will return an HTML `div` element that is styled according to the responsive design requirements.

Sample Input:

let responsiveDiv = createResponsiveDiv();

Sample Code:

function createResponsiveDiv() {

    let div = document.createElement('div');

    let style = document.createElement('style');

    style.innerHTML = `

        #responsiveDiv {

            width: 100%;

        }

        @media (min-width: 600px) {

            #responsiveDiv {

                width: 50%;

            }

        }

    `;

    document.head.appendChild(style);

    div.id = "responsiveDiv";

    return div;

}

Explanation:

The `createResponsiveDiv` function starts by creating a new `div` element. 

Next, it creates a `style` element and sets its innerHTML to the desired CSS. This CSS first sets the width of the `div` (which will be given the id “responsiveDiv”) to 100 percent. Then, it uses a media query to change this width to 50 percent if the width of the viewport is at least 600px. The style element is then appended to the `head` of the document.

Finally, the function gives the `div` the id “responsiveDiv” and returns it.

This question is the most challenging yet, requiring a strong understanding of media queries and how they can be used to create responsive designs. It also continues to test the developer’s skills in using JavaScript to create and manipulate HTML and CSS.

6. Implementing a Function to Apply a CSS Animation

This question evaluates a developer’s understanding of CSS animations, a sophisticated feature of CSS that’s fundamental to creating interactive and engaging web experiences. 

Task: Write a JavaScript function named `applyAnimation` that takes an HTML element’s id and applies a CSS keyframe animation to it. The animation should gradually change the element’s background color from red to blue over a period of 5 seconds.

Input Format: The function will take one argument: a string representing the id of an HTML element.

Constraints: The id will correspond to an existing HTML element.

Sample Input:

applyAnimation('myDiv');

Sample Code:

function applyAnimation(id) {

    let element = document.getElementById(id);

    let style = document.createElement('style');

    style.innerHTML = `

        @keyframes colorChange {

            0% {background-color: red;}

            100% {background-color: blue;}

        }

        #${id} {

            animation: colorChange 5s;

        }

    `;

    document.head.appendChild(style);

}

Explanation:

The `applyAnimation` function starts by getting a reference to the HTML element with the specified id using the `getElementById` method.

Next, it creates a `style` element and sets its innerHTML to define a CSS keyframe animation named `colorChange`. This animation gradually changes an element’s background color from red to blue. The CSS also applies this animation to the element with the specified id and sets the animation’s duration to 5 seconds. The style element is then appended to the `head` of the document.

This question significantly raises the difficulty level by requiring the candidate to use CSS keyframes, a complex but powerful feature that is crucial for creating animations in CSS. 

7. Creating a CSS Variable Manipulation Function

The final question explores the developer’s knowledge of CSS Variables (or custom properties), a more advanced feature of CSS. Custom properties provide a powerful way to create reusable values in CSS, which can be manipulated via JavaScript.

Task: Write a JavaScript function named `changeTheme` that takes two parameters: an HTML element’s id and a string representing a color. The function should change the value of the CSS variable `–theme-color` for the specified element to the provided color.

Input Format: The function will take two arguments: a string representing the id of an HTML element and another string representing a color.

Constraints:

• The id will correspond to an existing HTML element.
• The color will be a valid CSS color.

Sample Input:

changeTheme('myDiv', 'purple');

Sample Code:

function changeTheme(id, color) {

    let element = document.getElementById(id);

    element.style.setProperty('--theme-color', color);

}

Explanation:

The `changeTheme` function begins by getting a reference to the HTML element with the specified id.

Then, it uses the `setProperty` method to change the value of the CSS variable `–theme-color` for that element to the provided color.

This question represents a culmination of the candidate’s CSS and JavaScript knowledge, requiring them to understand how to manipulate CSS variables — a feature that brings a lot of power and flexibility to CSS. CSS variables can help reduce repetition, provide better scalability, and even allow for things like theme switching in CSS.

Resources to Improve HTML Knowledge

• HackerRank CSS Skills Certification
• HackerRank Interview
• Tech Interview Prep: How To Ace Your Interview

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