Vh/Vw vs. % in CSS: When Should You Use Viewport Units for Responsive Design?

Comparing vh/vw and % CSS Units

When discussing responsive design, it's essential to understand the differences between various CSS units. Among them, vh and vw have emerged as valuable tools for scaling elements relative to the viewport's dimensions.

vh and vw: Units of Viewport Dimensions

vh represents the percentage of the viewport's height, while vw represents the percentage of its width. These units enable elements to adjust dynamically based on the available viewport space. For instance, an element set to 50vh will occupy 50% of the viewport's height, regardless of the window's size.

Why Introduce vh/vw When % Exists?

While % also allows for percentage-based sizing, vh/vw offer a crucial distinction: They define percentages relative to the viewport specifically. This eliminates potential inconsistencies when nested elements and their parent containers have different heights.

Example: Parent vs. Viewport Height

Consider the following example:

.parent {
    height: 100px;
}

.child {
    height: 100%;
}

In this scenario, the child element will occupy 100% of its parent container's height, which is 100px.

Now, if we adjust the above CSS to use vh:

.parent {
    height: 100px;
}

.child {
    height: 100vh;
}

The child element's height will be 100% of the viewport's height, which may differ from the parent container's height. This distinction is particularly important for responsive layouts where elements should scale relative to the entire viewport.

Conclusion

While % and vh/vw can achieve similar results in certain scenarios, vh/vw provide a unique advantage by referencing the viewport's dimensions directly. This ensures that elements adjust consistently, regardless of nesting or container sizes, making them essential tools for responsive web design.

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