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Learn DVH CSS: Introductory Guide
Introduction: Embracing Dynamic Viewport Units
The web is increasingly accessed on a diverse range of devices, from towering desktop monitors to compact smartphones held in portrait or landscape mode. Responsive web design has become paramount, and one of the trickiest aspects of achieving true responsiveness lies in handling the ever-changing viewport – the visible area of the web page. Traditional CSS units like pixels (px), percentages (%), viewport width (vw), and viewport height (vh) offer good control, but they fall short when dealing with dynamic elements on mobile devices, particularly the presence or absence of browser UI elements like address bars and navigation bars.
Enter DVH (Dynamic Viewport Height) units, and their related siblings: lvh, svh, and dvh itself, along with corresponding width units dvw, lvw, and svw. These units provide a much more nuanced and accurate way to define dimensions relative to the viewport, taking into account the dynamic behavior of browser UI. This guide will provide a comprehensive introduction to these powerful units, explaining their purpose, behavior, and practical applications.
The Problem with Traditional Viewport Units (vh, vw)
Before diving into DVH units, let’s understand the limitations of the traditional vh and vw units. These units represent 1% of the viewport’s height and width, respectively. So, 100vh would theoretically be the full height of the viewport, and 50vw would be half its width.
This works perfectly fine on desktop browsers where the viewport is generally static. However, on mobile devices, the situation is more complex. Consider a typical scenario:
- Initial Load: A user loads a webpage on their mobile phone. The browser’s address bar is visible, taking up some space at the top of the screen.
- Scrolling Down: As the user scrolls down, the address bar (and sometimes a bottom navigation bar) might retract or minimize to provide more screen real estate for the content.
100vhIssue: If you’ve set an element’s height to100vh, it initially accounts for the address bar. When the address bar retracts, the element remains the same height, potentially extending beyond the visible area, creating unwanted scrolling or layout issues. The100vhvalue doesn’t dynamically adjust to the newly available space.
This inconsistency is what DVH units aim to solve. They provide a way to reference the actual available viewport height, considering the presence or absence of browser UI elements.
Introducing DVH Units: lvh, svh, and dvh
To address the shortcomings of vh, CSS introduced three new viewport height units:
lvh(Large Viewport Height): Represents 1% of the viewport height when all dynamic browser UI elements (like address bars) are retracted or minimized. This represents the largest possible visible area.svh(Small Viewport Height): Represents 1% of the viewport height when all dynamic browser UI elements are expanded or visible. This represents the smallest possible visible area.dvh(Dynamic Viewport Height): Represents 1% of the viewport height, and it dynamically updates as the browser UI elements expand or retract. This is the most flexible and responsive unit.
The corresponding width units behave similarly:
lvw(Large Viewport Width)svw(Small Viewport Width)dvw(Dynamic Viewport Width)
These units provide three distinct reference points for sizing elements:
- Worst-Case Scenario (
svh,svw): Your element will always be fully visible, even when the browser UI is taking up the most space. This is useful for critical elements that must always be in view. - Best-Case Scenario (
lvh,lvw): Your element will take full advantage of the available space when the browser UI is minimized. This is useful for maximizing immersive experiences. - Dynamic Adaptation (
dvh,dvw): Your element will smoothly adjust its size as the browser UI changes, providing the best balance between responsiveness and utilizing available space.
Practical Examples and Use Cases
Let’s illustrate the use of these units with practical examples:
1. Full-Screen Hero Section:
A common design pattern is to have a hero section that occupies the entire viewport height. Using 100vh can lead to issues on mobile, as described earlier. 100dvh provides a robust solution:
css
.hero {
height: 100dvh; /* Dynamically adjusts to viewport height */
background-image: url('hero-image.jpg');
background-size: cover;
background-position: center;
color: white;
display: flex;
align-items: center;
justify-content: center;
}
With 100dvh, the hero section will always fill the visible area, regardless of whether the address bar is showing or hidden.
2. Sticky Footer that Respects Browser UI:
A sticky footer should always be at the bottom of the visible viewport, not the bottom of the entire document. Using svh ensures this behavior:
“`css
body {
display: flex;
flex-direction: column;
min-height: 100dvh; / Ensure the body is at least the full dynamic viewport height /
}
.content {
flex: 1; / Grow to fill available space /
}
.footer {
height: 50px; / Fixed height for the footer /
background-color: #333;
color: white;
/ No need for positioning: absolute or fixed /
}
``min-height: 100dvh
In this improved example, theon thebodyensures that the content area will always fill at least the dynamically available screen height. The.contentdiv withflex: 1` will grow to fill any remaining space after the fixed-height footer, ensuring proper layout even when the address bar’s visibility toggles. The footer is a naturally positioned block element and will be ‘pushed’ down by the content.
3. Modal Dialogs:
When displaying a modal dialog, you often want it to be centered vertically and horizontally, and you might want it to take up a significant portion of the screen, but not necessarily the entire screen. svh and dvh can be used together effectively:
css
.modal {
position: fixed;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
width: 80vw;
max-height: 90svh; /* Maximum height based on the smallest viewport */
overflow-y: auto; /* Allow scrolling if content exceeds max-height */
background-color: white;
border: 1px solid #ccc;
padding: 20px;
}
Using max-height: 90svh ensures the modal will never be taller than 90% of the viewport height, even when the address bar is visible. This prevents the modal from being cut off at the bottom. The overflow-y: auto allows scrolling within the modal if its content exceeds the maximum height.
4. Avoiding Layout Shifts:
Layout shifts (CLS – Cumulative Layout Shift) are a core web vital that negatively impacts user experience and SEO. DVH units can help minimize layout shifts caused by the browser UI changing size. By using dvh for elements that are likely to be affected by the address bar, you ensure that the layout adapts smoothly instead of jumping.
5. Landscape Orientation Considerations
While dvh, lvh, and svh are primarily focused on height, the corresponding width units (dvw, lvw, svw) can be important in landscape mode on mobile devices. Some mobile browsers have UI elements that appear on the sides of the screen in landscape orientation, affecting the available width. Using dvw can ensure your layout adapts correctly in these situations.
Browser Compatibility and Fallbacks
DVH units are relatively new, but browser support is excellent and rapidly growing. As of late 2023, all major modern browsers (Chrome, Firefox, Safari, Edge) support them. However, it’s crucial to provide fallbacks for older browsers that don’t recognize these units.
There are several approaches to providing fallbacks:
1. Using vh as a Fallback:
The simplest approach is to define the vh value before the dvh value. Browsers that don’t understand dvh will simply ignore it and use the vh value.
css
.hero {
height: 100vh; /* Fallback for older browsers */
height: 100dvh; /* Preferred value for modern browsers */
}
This is generally a safe fallback, as vh provides a reasonable approximation in most cases. The layout might not be perfect on older mobile browsers, but it will still be functional.
2. Using @supports Feature Queries:
For more precise control, you can use @supports feature queries to detect support for DVH units and apply styles conditionally:
“`css
.hero {
height: 100vh; / Default height /
}
@supports (height: 100dvh) {
.hero {
height: 100dvh; / Apply DVH only if supported /
}
}
“`
This approach is more robust, as it explicitly checks for support before applying the dvh style.
3. Using a PostCSS Plugin (Autoprefixer):
If you’re using a build process with PostCSS, you can use a plugin like postcss-preset-env (which includes Autoprefixer) to automatically handle fallbacks for you. This is the recommended approach for larger projects, as it simplifies the process and ensures consistent fallback handling.
“`javascript
// postcss.config.js
module.exports = {
plugins: [
require(‘postcss-preset-env’)({
stage: 3, // Or a higher stage that includes DVH units
features: {
‘cascade-layers’: false, //if not using cascade layers
}
}),
],
};
``dvh
With this configuration, you can write your CSS usingunits, and the PostCSS plugin will automatically add the necessaryvh` fallbacks during the build process. You do not need to specify it separately in your source code.
4. JavaScript-Based Fallback (Not Recommended):
While possible, using JavaScript to detect browser support and dynamically set heights is generally not recommended for this specific scenario. It adds unnecessary complexity and can lead to performance issues. CSS-based solutions (using vh fallback or @supports) are much more efficient and maintainable. JavaScript should be a last resort.
Beyond Height and Width: Logical Viewport Units
The concept of dynamic viewport units extends beyond just dvh, svh, and lvh. There are also logical viewport units, which are based on the inline and block directions of the writing mode. These are particularly useful for websites that support multiple languages and writing modes (e.g., left-to-right, right-to-left, top-to-bottom).
dvi(Dynamic Viewport Inline): Equivalent todvwin left-to-right or right-to-left writing modes, but equivalent todvhin vertical writing modes.dvb(Dynamic Viewport Block): Equivalent todvhin left-to-right or right-to-left writing modes, but equivalent todvwin vertical writing modes.lvi(Large Viewport Inline),svi(Small Viewport Inline),lvb(Large Viewport Block),svb(Small Viewport Block): Similar logical variations forlvw,svw,lvh, andsvh.
These logical units ensure that your layout adapts correctly regardless of the writing mode, making your website more accessible and user-friendly for a global audience. For most Western language sites you’re unlikely to need these, however.
Combining DVH Units with Other CSS Techniques
DVH units are most effective when combined with other modern CSS techniques, such as:
- Flexbox and Grid Layout: These layout modules provide powerful tools for creating responsive and flexible layouts. DVH units can be used within Flexbox or Grid containers to control the size of individual items.
min(),max(), andclamp()Functions: These CSS functions allow you to set minimum, maximum, and preferred values for properties like width and height. You can combine them with DVH units for even greater control.
css
.element {
/* Height is at least 200px,
at most 500px,
and prefers 50dvh if it falls within that range */
height: clamp(200px, 50dvh, 500px);
}- Media Queries: While DVH units reduce the need for some media queries, they don’t eliminate them entirely. Media queries are still essential for making major layout changes based on screen size or orientation. DVH units can be used within media queries to fine-tune the layout for specific breakpoints.
Best Practices and Considerations
- Prioritize
dvh: In most cases,dvhis the best choice for responsiveness. It automatically adapts to the changing viewport height, providing a smooth user experience. - Use
svhfor Critical Elements: If you have elements that must be visible at all times, usesvhto ensure they’re sized based on the smallest possible viewport. - Use
lvhSparingly:lvhshould be used with caution, as it can result in elements being too large when the browser UI is visible. It’s best suited for situations where you want to maximize the use of space when the UI is hidden, such as in full-screen image galleries or video players. - Test Thoroughly: Always test your layouts on different devices and browsers, especially mobile devices, to ensure that DVH units are behaving as expected. Pay close attention to how the layout changes when the address bar appears and disappears.
- Consider Performance: While DVH units are generally performant, excessive use of
dvhon a very large number of elements could theoretically impact performance on some very low-powered devices. If you’re targeting such devices, profile your website’s performance and consider optimizing if necessary. In practice, this is rarely a significant concern. - Don’t Overuse
dvhunnecessarily. If a container’s size does not change, then using a static value will be more efficient.
Debugging DVH Unit Issues
If you encounter issues with DVH units, here are some debugging tips:
- Browser Developer Tools: Use your browser’s developer tools (usually accessed by pressing F12) to inspect the computed values of your elements. This will show you the actual height (in pixels) that the browser is assigning to elements with
dvh,svh, orlvhunits. - Toggle Address Bar: Manually toggle the address bar visibility (usually by scrolling up and down) to see how your layout responds. This can help you identify if the issue is related to the dynamic behavior of the viewport.
- Check for Fallbacks: If you’re using fallbacks, make sure they’re being applied correctly in older browsers. Use the
@supportsfeature query or inspect the computed styles in the developer tools. - Simplify Your Layout: If you’re having trouble isolating the issue, try simplifying your layout by temporarily removing other CSS properties that might be interfering.
- Check for parent container restrictions. If a parent container has a fixed height, this will impact how
dvhvalues are computed for children.
Looking Ahead: Container Queries and Viewport Units
Container queries are a newer CSS feature that allows you to style elements based on the size of their containing element, rather than the viewport. While container queries are incredibly powerful, they don’t replace DVH units. DVH units are still essential for situations where you need to size elements relative to the viewport itself, taking into account the dynamic behavior of browser UI. Container queries and DVH units are complementary tools that can be used together to create even more sophisticated and responsive layouts.
Recap: Dynamic Viewport Units
The introduction of Dynamic Viewport Height (DVH) units, including lvh, svh, and dvh, along with their corresponding width counterparts, represents a significant advancement in responsive web design. They address a critical limitation of traditional viewport units (vh, vw) by accounting for the dynamic behavior of browser UI elements on mobile devices. This allows for more precise control over layout and a smoother user experience, particularly when dealing with full-screen elements, sticky footers, and modals. By understanding the differences between these units and employing appropriate fallbacks for older browsers, web developers can create truly responsive designs that adapt gracefully to the ever-changing landscape of web-enabled devices. The key is to choose the right unit for the job: dvh for general responsiveness, svh for ensuring visibility, and lvh for maximizing space when appropriate. Combining DVH units with other modern CSS techniques like Flexbox, Grid, and clamp() provides a powerful toolkit for building robust and adaptable web layouts.