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home / developersection / interviews / what strategies help reduce cls on a mobile site?
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Anubhav Sharma
Anubhav Sharma

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The Anubhav portal was launched in March 2015 at the behest of the Hon'ble Prime Minister for retiring government officials to leave a record of their experiences while in Govt service .

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1 Answers

Anubhav Sharma
Anubhav Sharma 06 Apr 2025
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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.
<img src="banner.jpg" width="600" height="400" />

Or

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.
.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.
@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.
<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.
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