CSS Unit Battle: EMs vs REMs…FIGHT!

In the world of CSS, there are numerous units at our disposal for sizing elements, each with its own strengths and weaknesses. Two units that often spark debate among developers are the humble EM and the resolute REM. As a full-stack developer and professional coder, I‘ve had my fair share of battles with these units. Let‘s dive deep into the ring and explore the intricacies of EMs and REMs to determine which one reigns supreme!

The Contenders

Before we pit these units against each other, let‘s understand their origins and how they operate.

EM: The Relative Ruler

The EM unit, short for "ephemeral unit," has been around since the early days of typesetting. In CSS, 1em is equal to the computed font-size of an element. The catch? It‘s relative to the font-size of the element‘s parent. If an element has a font-size of 16px and its child is set to 2em, the child‘s computed font-size will be 32px (2 * 16px).

.parent {
  font-size: 16px;
}

.child {
  font-size: 2em; /* Computed font-size: 32px */
}

This relative nature of EMs allows for easy proportional sizing of elements. However, it also means that nested elements with EM sizes can compound, leading to unexpected results.

REM: The Root Ruler

The REM unit, introduced in CSS3, stands for "root em." Unlike EMs, REMs are always relative to the root element‘s (html) font-size. By default, browsers set the root font-size to 16px. So, an element with 2rem will have a computed size of 32px (2 * 16px), regardless of its parent‘s font-size.

html {
  font-size: 16px;
}

.element {
  font-size: 2rem; /* Computed font-size: 32px */
}

REMs offer a more predictable and consistent way to size elements. Changing the root font-size will proportionately scale all REM-based sizes throughout the document.

The Battle Rounds

Now that we know the contenders let‘s see how they fare in different scenarios.

Round 1: Inheritance and Predictability

In this round, we‘ll examine how EMs and REMs behave when inherited through nested elements.

Consider the following HTML structure:

<div class="grandparent">
  <div class="parent">
    <div class="child"></div>
  </div>
</div>

Let‘s apply some styles:

.grandparent {
  font-size: 16px;
}

.parent {
  font-size: 1.5em;
}

.child {
  font-size: 2em;
  padding: 1em;
}

In this case, the .parent element will have a computed font-size of 24px (1.5 16px), and the .child element will have a font-size of 48px (2 24px) and padding of 48px (1 * 48px).

If we were to change the .grandparent font-size to 20px, the .parent would become 30px (1.5 20px), and the .child would be 60px (2 30px) with 60px padding.

As you can see, EMs can create a cascading effect where sizes compound through nesting. This makes it harder to predict and control sizes, especially in complex layouts.

Now, let‘s see how REMs handle the same scenario:

html {
  font-size: 16px;
}

.grandparent {
  font-size: 1rem;
}

.parent {
  font-size: 1.5rem;
}

.child {
  font-size: 2rem;
  padding: 1rem;
}

With REMs, the .grandparent will have a font-size of 16px (1 16px), the .parent 24px (1.5 16px), and the .child 32px (2 16px) with 16px padding (1 16px).

Changing the root font-size to 20px will scale all sizes proportionately: .grandparent to 20px, .parent to 30px, and .child to 40px with 20px padding.

REMs offer a more stable and predictable sizing system, as they are always based on the root font-size. This makes it easier to maintain consistent sizing and scale elements globally.

Winner: REM takes this round for its predictability and resistance to compounding issues.

Round 2: Flexibility and Contextual Sizing

While REMs excel at consistent sizing, EMs have a unique strength in their ability to size elements relative to their context.

Imagine a card component with a title, description, and icon:

<div class="card">
  <h2 class="title">Card Title</h2>
  <p class="description">Card description goes here.</p>
  <img src="icon.png" alt="Card Icon" class="icon" />
</div>

We want the icon to always be 1.5 times the size of the title. With EMs, we can achieve this easily:

.card {
  font-size: 16px;
}

.title {
  font-size: 1.5em;
}

.icon {
  width: 1.5em;
  height: 1.5em;
}

Now, the icon will always scale proportionately with the title, even if the .card font-size changes. This flexibility is particularly useful for creating responsive and adaptive components.

To achieve the same effect with REMs, we would need to use a multiplier based on the root font-size:

html {
  font-size: 16px;
}

.card {
  font-size: 1rem;
}

.title {
  font-size: 1.5rem;
}

.icon {
  width: 1.5rem;
  height: 1.5rem;
}

While this works, it‘s less intuitive and requires updating the multiplier if the root font-size changes.

Winner: EM takes this round for its flexibility in contextual sizing.

Round 3: Media Queries and Responsive Design

Media queries are a crucial tool for creating responsive designs that adapt to different screen sizes. Let‘s see how EMs and REMs stack up in this context.

Consider a typical media query for adjusting font sizes on smaller screens:

html {
  font-size: 16px;
}

@media screen and (max-width: 768px) {
  html {
    font-size: 14px;
  }
}

.title {
  font-size: 2rem;
}

.subtitle {
  font-size: 1.5rem;
}

In this case, using REMs makes it straightforward to scale down the font sizes at the specified breakpoint. The .title will be 32px (2 16px) on larger screens and 28px (2 14px) on smaller ones, while the .subtitle will be 24px (1.5 16px) and 21px (1.5 14px), respectively.

If we were to use EMs instead:

.container {
  font-size: 16px;
}

@media screen and (max-width: 768px) {
  .container {
    font-size: 14px;
  }
}

.title {
  font-size: 2em;
}

.subtitle {
  font-size: 1.5em;
}

The font sizes would still scale down, but the change would be localized to the .container element. This can be useful for component-specific responsive adjustments.

However, REMs have an advantage when it comes to global responsive sizing. By changing the root font-size at different breakpoints, you can efficiently scale all REM-based sizes throughout the document.

Winner: REM takes this round for its simplicity in global responsive sizing, but EM put up a good fight with its localized responsiveness.

Round 4: Accessibility and User Experience

Accessibility is a critical consideration in web development, and the choice of units can impact user experience.

One of the benefits of using relative units like EMs and REMs is that they respect user preferences for font sizes. If a user has set a larger default font size in their browser, elements sized with EMs or REMs will scale accordingly, ensuring readability.

However, REMs have a slight edge in this regard. Since they are always relative to the root font-size, they provide a more consistent and predictable experience. Users can easily adjust the global font size without worrying about unintended layout shifts caused by compounding EMs.

Furthermore, using REMs for sizing can help maintain a balanced and harmonious design across different devices and platforms. By relying on a single source of truth (the root font-size), designers can create layouts that scale proportionately and maintain their intended ratios.

Winner: REM takes this round for its consistency and predictability in maintaining accessible and user-friendly designs.

The Verdict

After four intense rounds, it‘s time to declare a winner. But wait! It turns out that EMs and REMs are not bitter rivals but rather complementary partners in the world of CSS sizing.

EMs excel at contextual sizing and provide flexibility for component-level responsiveness. They are particularly useful when you want elements to scale proportionately within a specific context.

REMs, on the other hand, offer a more predictable and consistent sizing system. They are ideal for global scaling, responsive design, and maintaining accessibility across devices.

In practice, a combination of both units often yields the best results. You can use REMs for overall layout sizing and EMs for fine-tuning individual components. This approach gives you the benefits of both units while mitigating their drawbacks.

As a full-stack developer and professional coder, my recommendation is to start with REMs as your default sizing unit and use EMs strategically where contextual sizing is needed. By leveraging the strengths of both units, you can create designs that are flexible, responsive, and accessible.

Best Practices and Tips

To make the most of EMs and REMs in your projects, consider the following best practices and tips:

1. Set a default font-size on the root element: When using REMs, ensure that you set a default font-size on the html element. A common value is 16px, which is the default in most browsers. This serves as the base for all REM calculations.

2. Use a CSS preprocessor: Preprocessors like Sass or Less allow you to define variables for your font sizes and other values. This makes it easier to maintain and update your styles across the project.

3. Embrace the power of calc(): The calc() function in CSS allows you to combine units and perform mathematical operations. You can use it to create complex sizing relationships, such as "font-size: calc(1rem + 0.5vw)," which scales with both the root font-size and the viewport width.

4. Maintain a type scale: Establish a consistent typographic scale using relative units. For example, you can use a scale like 1rem, 1.25rem, 1.5rem, 2rem, etc., to create a harmonious hierarchy of font sizes.

5. Test on different devices and browsers: Always test your designs on various devices, screen sizes, and browsers to ensure that your sizing works as intended. Pay attention to accessibility and readability across different configurations.

The Future of CSS Units

As CSS evolves, new units and techniques emerge to address the challenges of responsive and adaptive design. Some notable developments include:

• Viewport units (vw, vh, vmin, vmax): These units allow sizing relative to the viewport dimensions, enabling truly responsive layouts.
• CSS Grid and Flexbox: Modern layout systems like Grid and Flexbox provide powerful tools for creating flexible and responsive designs without relying heavily on specific units.
• CSS Variables (Custom Properties): CSS Variables allow you to define reusable values and easily update them throughout your stylesheets, similar to variables in preprocessors.

While EMs and REMs remain essential tools in a developer‘s toolkit, it‘s important to stay updated with the latest CSS advancements and adapt your practices accordingly.

Conclusion

In the battle of CSS units, EMs and REMs have proven to be formidable contenders, each with its own strengths and weaknesses. By understanding their behavior and appropriate use cases, you can harness their power to create designs that are flexible, responsive, and accessible.

Remember, the key to mastering CSS sizing is to use the right unit for the job at hand. Don‘t be afraid to mix and match EMs, REMs, and other units to achieve your desired outcome.

As a full-stack developer and professional coder, your goal should be to create designs that not only look great but also provide an optimal user experience across a wide range of devices and platforms. By leveraging the power of EMs and REMs, along with other CSS tools and techniques, you can craft designs that are both beautiful and functional.

So, go forth and embrace the world of relative units! Experiment, learn, and create designs that adapt and scale with ease. May your CSS be responsive, your layouts flexible, and your users delighted!