---
title: "What strategies help reduce CLS on a mobile site?"  
description: "What strategies help reduce CLS on a mobile site?"  
author: "Anubhav Sharma"  
published: 2025-04-06  
updated: 2025-04-06  
canonical: https://www.mindstick.com/interview/34028/what-strategies-help-reduce-cls-on-a-mobile-site  
category: "web development"  
tags: ["web development"]  
reading_time: 4 minutes  

---

# What strategies help reduce CLS on a mobile site?

Reducing **Cumulative Layout Shift (CLS)** is crucial on mobile where screen space is limited and layout shifts are extra jarring. CLS measures how much content unexpectedly moves during load. Here's how to keep your layout stable:

#### 1. Always Set Width & Height on Images and Videos

- This prevents the browser from reserving the correct space before loading.
- Use `width` and `height` attributes or `aspect-ratio` in CSS.

```html
<img src="banner.jpg" width="600" height="400" />
```

Or

```css
img {
  aspect-ratio: 3 / 2;
  width: 100%;
  height: auto;
}
```

#### 2. Reserve Space for Ads, Embeds, and Iframes

- Dynamically injected ads or embeds (like YouTube) often cause shifts.
- Use CSS to give them a placeholder box of fixed height.

```css
.ad-slot {
  min-height: 250px;
}
```

#### 3. Avoid Inserting Content Above Existing Content (unless triggered by user)

- E.g., banners, cookie notices, toolbars.
- If you must show them, overlay them (position: fixed) instead of pushing content down.

#### 4. Use Font Display Strategies to Prevent Flash of Unstyled Text (FOUT)

- Use `font-display: swap;` in your `@font-face` to avoid layout shifts due to late font loading.

```css
@font-face {
  font-family: 'CustomFont';
  src: url('font.woff2') format('woff2');
  font-display: swap;
}
```

#### 5. Preload Key Fonts & Images

- Especially fonts or hero images above the fold.
- Prevents late loading that causes reflows.

```html
<link rel="preload" as="font" href="font.woff2" type="font/woff2" crossorigin>
```

#### 6. Minimize Layout Thrashing in JS

- Avoid JS that adds/removes DOM elements without space planning.
- Batch DOM updates and avoid animations that change layout (use `transform` instead of `top/left`).

#### 7. Use Lighthouse or Chrome DevTools to Audit CLS

- In DevTools → Performance → Layout Shifts.
- Identify what’s moving and when.

#### Mobile-specific tips:

- Be extra careful with fluid layouts, media queries, and viewport resizing.
- Don't lazy-load content that’s immediately visible.

## Answers

### Answer by Anubhav Sharma

Reducing **Cumulative Layout Shift (CLS)** is crucial on mobile where screen space is limited and layout shifts are extra jarring. CLS measures how much content unexpectedly moves during load. Here's how to keep your layout stable:

#### 1. Always Set Width & Height on Images and Videos

- This prevents the browser from reserving the correct space before loading.
- Use `width` and `height` attributes or `aspect-ratio` in CSS.

```html
<img src="banner.jpg" width="600" height="400" />
```

Or

```css
img {
  aspect-ratio: 3 / 2;
  width: 100%;
  height: auto;
}
```

#### 2. Reserve Space for Ads, Embeds, and Iframes

- Dynamically injected ads or embeds (like YouTube) often cause shifts.
- Use CSS to give them a placeholder box of fixed height.

```css
.ad-slot {
  min-height: 250px;
}
```

#### 3. Avoid Inserting Content Above Existing Content (unless triggered by user)

- E.g., banners, cookie notices, toolbars.
- If you must show them, overlay them (position: fixed) instead of pushing content down.

#### 4. Use Font Display Strategies to Prevent Flash of Unstyled Text (FOUT)

- Use `font-display: swap;` in your `@font-face` to avoid layout shifts due to late font loading.

```css
@font-face {
  font-family: 'CustomFont';
  src: url('font.woff2') format('woff2');
  font-display: swap;
}
```

#### 5. Preload Key Fonts & Images

- Especially fonts or hero images above the fold.
- Prevents late loading that causes reflows.

```html
<link rel="preload" as="font" href="font.woff2" type="font/woff2" crossorigin>
```

#### 6. Minimize Layout Thrashing in JS

- Avoid JS that adds/removes DOM elements without space planning.
- Batch DOM updates and avoid animations that change layout (use `transform` instead of `top/left`).

#### 7. Use Lighthouse or Chrome DevTools to Audit CLS

- In DevTools → Performance → Layout Shifts.
- Identify what’s moving and when.

#### Mobile-specific tips:

- Be extra careful with fluid layouts, media queries, and viewport resizing.
- Don't lazy-load content that’s immediately visible.


---

Original Source: https://www.mindstick.com/interview/34028/what-strategies-help-reduce-cls-on-a-mobile-site

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