Member Avatar for salm_27

One of my client's micro <link snipped> site has some cls issues and that's why it has a slow response on mobiles. How can we resolve cls issues.

Edited by Dani because: Link snipped because it's irrelevant to the question
  • 3 Contributors
  • 4 Replies
  • 250 Views
  • 2 Days Discussion Span
  • Latest Post Latest Post by Dani

Dani AI

Generated

Quick clarification: as noted, CLS (Cumulative Layout Shift) measures unexpected visual movement, not raw network latency. Mobile users often describe layout jumps as "slowness" because content reflows while other resources load, but slow TTFB/LCP and high CLS can both be present. Common CLS sources: images/iframes without intrinsic size, lazy-loading without reserved space, third‑party widgets and ads that inject content, and web fonts that change metrics after swap.

Practical diagnosis (fast workflow):

  • Capture field data (Search Console Core Web Vitals or CrUX) and run lab checks with Lighthouse/PageSpeed Insights using mobile throttling.
  • In Chrome DevTools: Performance → record page load, enable the Layout Shift Regions overlay, then inspect individual Layout Shift events to see which selectors moved.
  • Compare lab vs field: some shifts only appear in real-user sessions (ads, consent banners, slow networks).

Concrete fixes and examples to apply immediately:

  • Give media intrinsic sizes or an aspect ratio. Example HTML:
    <img src="/images/hero.jpg" alt="" width="1200" height="800" loading="lazy">
  • Use CSS aspect-ratio or the padding-top trick for responsive placeholders:
    
.media { aspect-ratio: 16/9; background:#eee; }

/ fallback /
.media-wrap { position:relative; width:100%; padding-top:56.25%; }
.media-wrap img { position:absolute; inset:0; width:100%; height:100%; }

- Reserve ad/iframe slots with fixed dimensions and a neutral skeleton (.ad-slot { width:300px; height:250px; min-height:250px; }).
- For fonts, preload critical faces and use a fallback stack with similar metrics or `font-display: swap` to reduce shifts:

<link rel="preload" href="/fonts/myfont.woff2" as="font" type="font/woff2" crossorigin>



Cautions and targets: preloading fonts increases bandwidth; reserved ad space can show empty areas—use neutral placeholders. Aim for CLS ≤ 0.10 for a good user experience and re-run both lab and field checks after each change to confirm improvements.

Recommended Answers

All 4 Replies

Member Avatar for rproffitt

What is "cls"? From memory it's the clearscreen command found in old TRS-80 Basic.

Member Avatar for Dani

What is "cls"? From memory it's the clearscreen command found in old TRS-80 Basic.

In the context of SEO, CLS (cumulative layout shift) is a ratio that is representative of how much content on a webpage "jumps around" after the initial paint happens. It's considered poor usability for someone to start reading some text, and suddenly have that text pushed down as an ad loads, for example.

You can improve CLS by ensuring that all external resources outside of the HTML page (images, ads, etc.) all have fixed dimensions specified either in the HTML or CSS, so that there is room already accounted for them as they load in place over slow mobile connections.

That being said, the website you pointed me to has a CLS of zero, meaning that there are no more issues.

Member Avatar for rproffitt

Why using acronyms, character case is important as well as subject. You can guess how acronyms are often a cause of miscommunication. I try to avoid them or define them as I write my papers and documentation.

Member Avatar for Dani

In the context of SEO (as this is the SEO forum), it’s absolutely always referred to as CLS. I actually had to look up what it stands for to give my explanation. One of those abbreviations that ends up becoming the primary word while the original spelled out version becomes obscure.

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.