CSS : Cascading Style Sheets

CSS is the language we use to style a Web page.

What is CSS?

CSS stands for Cascading Style Sheets
CSS describes how HTML elements are to be displayed on screen, paper, or in other media
CSS saves a lot of work. It can control the layout of multiple web pages all at once
External stylesheets are stored in CSS files

Why Use CSS?

CSS is used to define styles for your web pages, including the design, layout and variations in display for different devices and screen sizes.

CSS Example

body {

background-color: lightblue;
}

h1 {

color: white;

text-align: center;

}

p {

font-family: verdana;

font-size: 20px;

}

CSS Solved a Big Problem

HTML was NEVER intended to contain tags for formatting a web page!

HTML was created to describe the content of a web page, like:

<h1>This is a heading</h1>

<p>This is a paragraph.</p>

When tags like <font>, and color attributes were added to the HTML 3.2 specification, it started a nightmare for web developers. Development of large websites, where fonts and color information were added to every single page, became a long and expensive process.

To solve this problem, the World Wide Web Consortium (W3C) created CSS.

CSS removed the style formatting from the HTML page!

CSS Saves a Lot of Work!

The style definitions are normally saved in external .css files.

With an external stylesheet file, you can change the look of an entire website by changing just one file!

CSS interview questions to hire skilled web developers

Below is a list of the best interview questions to evaluate a candidate’s CSS skills. The questions cover various difficulty levels, from basic to advanced, and include sample answers to help you understand the responses, even if you’re not familiar with CSS.

1. What are selectors in CSS? Can you give an example?

Selectors are patterns used to select the element or elements you want to style. They can be used to find elements based on their name, ID, class, attributes, and more. For example:

p is a type selector that selects all <p> elements.

.navigation is a class selector that selects all elements with a class of navigation

#header is an ID selector that selects the element with the ID header

2. Explain the difference between classes and IDs.

In CSS, classes and IDs are selectors you can use to apply styles, but they’re different in terms of specificity and reusability:

Classes are best used when you need to style multiple elements similarly. They’re reusable and apply to multiple elements on a page. For example, you could use .button for all buttons on a page.

IDs are used for unique elements that appear once on any given page. IDs have higher specificity than classes, meaning styles applied with an ID override those applied with a class. For example, #navbar would be unique to the navigation bar.

3. What does “cascading” in Cascading Style Sheets mean?

“Cascading” in CSS describes the priority scheme that determines which style to apply to an element. It’s based on three main factors: specificity, importance, and source order.

Styles defined later in the CSS file or in a style sheet will override earlier ones if they have the same specificity. This system allows for styles to “cascade” down through layers of specificity and inheritance, giving developers control over the styling of web elements.

4. What are pseudo-classes? Give me a few examples.

Pseudo-classes are keywords added to selectors that specify a special state of the selected elements. They can be used to define a special behavior for an element in a particular state. For example:

:hover applies styles when the user hovers over an element

:focus applies styles when an element has focus

:nth-child(n) applies styles to the nth child of a parent

:checked applies styles to input elements that are checked, like checkboxes or radio buttons

5. How do you handle browser-specific styling issues?

Front-end developers usually handle browser-specific styling issues using:

Prefixes for CSS properties that require them, such as -webkit- for Chrome and

-moz- for Firefox

Conditional comments, primarily for older versions of Internet Explore

Feature detection libraries like Modernizr to apply styles if certain features are supported

Fallbacks, which provide alternative styles in case a browser does not support certain properties

CSS Resets, which normalize the styling across different browsers

6. How do you optimize your CSS for performance?

There are few strategies you can use to optimize performance in CSS, such as:

Minification: Reducing file size by removing unnecessary spaces, comments, and characters

Concatenation: Combining multiple CSS files into one to reduce HTTP requests

Using shorthand properties: Reducing the amount of code

Optimizing CSS selectors: Writing efficient selectors that reduce browser workload

Using CSS Sprites: Combining many images into one to reduce HTTP requests

Avoiding excessive use of CSS resets: Resets can be heavy, so it's better to use them only when necessary

7. Explain the concept of CSS frameworks. Can you name a few?

Knowledgeable candidates will explain that CSS frameworks are pre-written libraries that make it easier to design web layouts and components. They provide consistent styling across browsers and include reusable components, which speeds up the development process. Popular CSS frameworks include Bootstrap, Foundation, Tailwind CSS, and Bulma.

Looking for someone with top Bootstrap skills? Use our Bootstrap test to assess candidates.

8. How do you handle CSS for retina-ready displays?

To accommodate retina-ready displays, web developers can:

Use high-resolution media queries to display higher quality images

Use vector graphics like SVGs which are resolution-independent and scale without losing quality

Set CSS properties like background-size to ensure images scale correctly

Use modern image formats such as WebP for better compression and quality compared to traditional formats like JPEG

9. What new features does CSS3 have?

CSS3 introduced a few features for better web design, including:

Rounded corners

Box shadows and text shadows

Smoother transitions and animations without JavaScript

Multiple background images

New layout features, including Flexbox

Linear and radial gradients for smooth transitions between colors

10. How can you implement CSS transitions?

CSS transitions allow elements to change properties smoothly over a given timeframe.

To implement a transition, you need to specify the CSS property you want to transition, the duration of the transition, and optionally, the timing function and delay. For example:

div {

transition: background-color 0.5s ease-in-out;

}

div:hover { background-color: blue; }

This CSS would cause the div's background color to change to blue over a half-second period when hovered.

11. What are keyframe animations? How are they different from transitions?

Keyframe animations enable more complex animations than transitions. Transitions apply a single change between two states, while animations can include multiple states at different timings using @keyframes.

12. What are SVGs? How do you use them with CSS?

SVG (Scalable Vector Graphics) is an XML-based image format for two-dimensional graphics that support interactivity and animation. SVGs are resolution-independent, which makes them ideal for responsive design. Plus, they display well on high-resolution displays.

In CSS, SVGs can be styled with properties like fill, stroke, and stroke-width. You can embed SVG directly in HTML or reference it as an image source and manipulate it with CSS for dynamic styling and effects.

13. How can you use CSS to ensure accessibility in web design?

To improve web accessibility with CSS, designers can:

Use scalable units (like em or rem) for font sizes to support users who need larger text

Ensure sufficient color contrast between text and backgrounds

Use media queries to adapt layouts to different conditions, like reduced motion or high-contrast modes

Hide elements visually without removing them from screen readers

Provide focus styles for interactive elements to for easier keyboard navigation

14. How would you approach fixing a CSS bug that appears in only one web browser?

To tackle a browser-specific CSS bug, a developer would need to:

Identify the bug with the help of testing tools and browser developer tools

Check if it’s a known issue with documented workarounds

Use conditional comments or feature detection (like Modernizr) to apply specific styles or fixes

Isolate the problematic code and experiment with alternative CSS properties or values to achieve a similar effect

Validate your CSS to ensure no errors are causing unexpected behavior

15. Describe how you would create a responsive navigation menu.

Describe-how-you-would-create-a-responsive-navigation-menu graphic

The answers of skilled candidates should feature a few approaches. Web developers can build a navigation menu using:

Flexbox or CSS Grid: These models adjust to different screen sizes, making it easier to design responsive menus

Media queries: This enables changing the navigation layout at different breakpoints, i.e. switching from a horizontal to a dropdown menu on mobile devices

JavaScript or CSS for interactive elements: Toggling the menu on smaller screens using a hamburger icon

Accessible markup: Ensuring that the menu is usable with keyboard and screen readers

16. What steps would you take to convert a fixed layout to a responsive design?

Responsive design is key for providing a positive user experience – and any web designer and developer should be able to convert a fixed layout to a responsive page. Here are the steps they should take:

Implement fluid grids: Replace fixed-width dimensions with relative units like percentages or viewport units to ensure the layout adjusts to different screen sizes.

Use flexible images and media: Apply max-width: 100%; and height: auto; to images to make them scale within their containing elements

Use media queries: Apply different CSS styles based on device characteristics like screen width, resolution, and orientation

Update navigation: Adapt navigation elements to be touch-friendly and suitable for smaller screens, potentially using a hamburger menu for smaller devices

Test on multiple devices and screen sizes: Check whether the layout performs well on various devices

17. How can you use CSS to make a page load faster?

To improve a website’s loading speed with CSS, developers can:

Remove unnecessary styles, comments, and whitespace

Combine and compress CSS files to reduce HTTP requests and file size

Use shorthand notation to reduce the size of the CSS code

Use CSS Sprites to combine many small images into a single file to reduce HTTP requests

Prioritize above-the-fold content using media queries to load critical CSS first

Use modern CSS properties like transform over older properties like top and left for animations

18. How would you implement a custom checkbox using only CSS?

Here’s how to create a custom checkbox with CSS:

HTML code:

CSS code:

.custom-checkbox input[type="checkbox"] { visibility: hidden; }

.custom-checkbox .checkmark { height: 20px; width: 20px;

.background-color: #eee; display: inline-block; }

.custom-checkbox input[type="checkbox"]:checked + .checkmark { background-color: blue; }

This CSS hides the default checkbox and uses a span to create a custom checkmark. The :checked selector is used to change the checkbox’s appearance when it’s checked.

20. What are blend modes in CSS? How can you use them in web design?

CSS blend modes enable developers to specify blending effects between elements' background and their content or between an element and its background.

This is useful when manipulating colors, ensuring text visibility over different backgrounds, and creating visual themes. Skilled candidates will mention that blend modes are specified using the background-blend-mode or mix-blend-mode properties.

21. How do you handle DPI scaling in CSS?

Top applicants will know that to manage dots-per-inch (DPI) scaling, they can use responsive images with the srcset attribute, enabling the browser to select appropriate image resolutions based on the user device's DPI.

Alternatively, they can use vector graphics like SVGs, which scale well and provide crisp visuals at any resolution, or media queries that target high-resolution displays to increase font sizes.

22. What are some common security issues with CSS?

CSS has a low risk profile but skilled developers will know that some issues might still arise with it. Examples include:

CSS injection: Hackers can inject CSS into a page, altering its layout or using

background-url to load malicious content

Clickjacking: Transparent or opaque layers could trick users into clicking on elements they don't want to interact with

Security policies: Incorrectly configured content security policies (CSP) might allow attackers to bypass other security features

If you need someone with top cybersecurity skills, you can use a Cybersecurity test to further assess candidates’ technical knowledge of common web and network security threats.

23. Why are CSS style guide methodologies like BEM, SMACSS, and OOCSS important?

CSS methodologies like BEM (Block Element Modifier), SMACSS (Scalable and Modular Architecture for CSS), and OOCSS (Object-Oriented CSS) provide guidelines and patterns for writing CSS that improve the code’s scalability, maintainability, and modularity.

These methodologies promote top CSS best practices such as:

Reducing redundancy and increasing reusability

Easing maintenance and scalability by structuring code logically

Reducing specificity wars and improving performance

Use our Clean Code test to hire web developers who are able to write clean and easy to maintain code.

24. What is the difference between ::before and :before?

Top candidates will know that ::before is the correct and modern notation for CSS3 pseudo-elements and point out that :before was used in CSS2. The double-colon syntax (::) distinguishes pseudo-elements from pseudo-classes like :hover. Otherwise, both perform the same function.

25. How do you use CSS to handle text overflow in a container?

To manage text overflow in a container, a skilled developer would use the overflow, text-overflow, and white-space properties in combination:

.container { width: 200px; overflow: hidden; white-space: nowrap; text-overflow: ellipsis; }

This CSS will prevent the text from wrapping onto new lines (white-space: nowrap), cut off overflow (overflow: hidden), and display an ellipsis (text-overflow: ellipsis) to indicate truncated content.

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