Viewport units in CSS - Performance & Optimization

Performance: Viewport units

MEDIUM IMPACT

Viewport units affect the layout and sizing of elements relative to the browser window size, impacting rendering and layout recalculations on resize.

Sizing elements responsively to viewport size

CSS

div { height: 100dvh; } /* dynamic viewport height */

100dvh accounts for dynamic viewport changes, reducing layout shifts and reflows on mobile.

📈 Performance Gainreduces reflows and CLS by stabilizing height during viewport changes

Sizing elements responsively to viewport size

CSS

div { height: 100vh; } /* full viewport height */

100vh can cause layout shifts on mobile browsers when address bars show/hide, triggering reflows and CLS.

📉 Performance Costtriggers multiple reflows on viewport resize, causing noticeable layout shifts

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Using 100vh for height	Minimal	Multiple on resize	Medium	[X] Bad
Using 100dvh for height	Minimal	Single or none on resize	Low	[OK] Good

Rendering Pipeline

Viewport units are calculated during style calculation and layout stages. Changes in viewport size cause recalculation of styles and layout, triggering reflows and repaints.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckLayout (reflow) due to viewport size changes affecting element dimensions

Core Web Vital Affected

CLS

Viewport units affect the layout and sizing of elements relative to the browser window size, impacting rendering and layout recalculations on resize.

Optimization Tips

1Avoid using 100vh on mobile to prevent layout shifts.

2Use dynamic viewport units like 100dvh for better visual stability.

3Test viewport unit usage by resizing browser and checking for reflows.

Performance Quiz - 3 Questions

Which viewport unit helps reduce layout shifts on mobile browsers?

A 100dvh

B 100vh

C 100vw

D 100vmin

DevTools: Performance

How to check: Record a performance profile while resizing the browser window and observe layout and paint events.

What to look for: Look for frequent layout (reflow) events and layout shift markers indicating CLS issues.

Practice

1. What does the CSS unit 1vw represent?

easy

A. 1% of the viewport's width

B. 1% of the viewport's height

C. 1 pixel

D. 1% of the parent element's width

Solution

1. Step 1: Understand viewport width unit

   The unit vw stands for viewport width, so 1vw equals 1% of the browser window's width.

2. Step 2: Compare with other units

   Unlike vh which is viewport height, vw relates only to width, not height or pixels.

3. Final Answer:

   1% of the viewport's width -> Option A

4. Quick Check:

   1vw = 1% viewport width [OK]

Hint: Remember vw = viewport width percent [OK]

Common Mistakes:

• Confusing vw with vh
• Thinking vw is pixels
• Mixing viewport units with parent size

2. Which of the following is the correct CSS syntax to set an element's height to 50% of the viewport height?

easy

A. height: 50vw;

B. height: 50vmax;

C. height: 50vmin;

D. height: 50vh;

Solution

1. Step 1: Identify viewport height unit

   The unit vh means viewport height, so 50vh means 50% of the viewport height.

2. Step 2: Check other units

   vw is viewport width, vmin is the smaller of width or height, and vmax is the larger. Only vh sets height relative to viewport height directly.

3. Final Answer:

   height: 50vh; -> Option D

4. Quick Check:

   Use vh for viewport height in CSS [OK]

Hint: Use vh for height, vw for width [OK]

Common Mistakes:

• Using vw for height
• Confusing vmin and vmax
• Forgetting semicolon in CSS

3. Given this CSS:

div {
  width: 10vw;
  height: 20vh;
}

If the browser window is 1000px wide and 800px tall, what will be the div's width and height in pixels?

medium

A. Width: 200px, Height: 400px

B. Width: 10px, Height: 20px

C. Width: 100px, Height: 160px

D. Width: 1000px, Height: 800px

Solution

1. Step 1: Calculate width from vw

   10vw means 10% of viewport width. 10% of 1000px = 100px.

2. Step 2: Calculate height from vh

   20vh means 20% of viewport height. 20% of 800px = 160px.

3. Final Answer:

   Width: 100px, Height: 160px -> Option C

4. Quick Check:

   vw and vh convert to % of viewport size [OK]

Hint: Multiply vw/vh % by viewport pixels [OK]

Common Mistakes:

• Mixing width and height values
• Calculating percentages incorrectly
• Confusing vh with vw

4. This CSS code is intended to make a box fill the smaller dimension of the viewport, but it doesn't work as expected:

.box {
  width: 50vmin;
  height: 50vmin;
}

What is the likely problem?

medium

A. The viewport units need a unit like px after vmin

B. The viewport size might be changing, causing unexpected results

C. vmin is not supported by browsers

D. The CSS syntax is correct; problem is elsewhere

Solution

1. Step 1: Understand vmin behavior

   vmin uses the smaller of viewport width or height. If viewport changes size (like resizing window), the box size changes too.

2. Step 2: Identify dynamic viewport effect

   Because viewport size can change, the box size changes dynamically, which may look like it doesn't work as expected.

3. Final Answer:

   The viewport size might be changing, causing unexpected results -> Option B

4. Quick Check:

   vmin depends on viewport size changes [OK]

Hint: Remember viewport units react to window resizing [OK]

Common Mistakes:

• Thinking vmin needs px unit
• Assuming viewport units are fixed
• Believing vmin is unsupported

5. You want a square element that always fits inside the viewport without scrolling, using viewport units. Which CSS rule ensures the square's size adapts to the smaller viewport dimension?

hard

A. width: 100vmin; height: 100vmin;

B. width: 100vw; height: 100vh;

C. width: 100vmax; height: 100vmax;

D. width: 100%; height: 100%;

Solution

1. Step 1: Understand vmin and vmax

   vmin is the smaller of viewport width or height, vmax is the larger.

2. Step 2: Choose unit for fitting inside viewport

   To fit inside viewport without scrolling, use vmin so the square fits the smaller dimension.

3. Step 3: Confirm width and height match

   Setting both width and height to 100vmin creates a square that fits inside viewport.

4. Final Answer:

   width: 100vmin; height: 100vmin; -> Option A

5. Quick Check:

   Use vmin for square fitting smaller viewport side [OK]

Hint: Use vmin for size based on smaller viewport side [OK]

Common Mistakes:

• Using vmax causes overflow
• Using 100% depends on parent size
• Confusing vw/vh with vmin/vmax