Hvad er CLS (Cumulative Layout Shift)?
Hurtig definition
CLS måler den samlede mængde uventede layoutskift, der opstår i løbet af en sides levetid, og kvantificerer, hvor meget synligt indhold bevæger sig.
Cumulative Layout Shift (CLS) is a Core Web Vital that measures visual stability. It quantifies how much the content on a page moves around unexpectedly as it loads. A layout shift occurs when a visible element changes its position from one rendered frame to the next without user interaction.
A good CLS score is under 0.1. Between 0.1 and 0.25 needs improvement, and above 0.25 is poor. The score is calculated by multiplying the impact fraction (how much of the viewport was affected) by the distance fraction (how far elements moved).
Common causes of layout shift include images and videos without dimensions (the browser doesn't know how much space to reserve until they load), ads or embeds that load dynamically and push content down, web fonts that cause text to resize when they load (Flash of Unstyled Text), and content that's dynamically injected above existing content.
Fixing CLS typically involves always including width and height attributes on images and videos, reserving space for ads and dynamic content, using font-display: swap or optional for web fonts with preloading, and avoiding inserting content above existing content. CSS properties like aspect-ratio and min-height can also help reserve space.
Hvorfor det er vigtigt
Layout shifts are one of the most frustrating user experiences on the web. You're about to click a link, and suddenly the content jumps because an ad loaded. You're reading an article, and the text shifts because an image rendered. These unexpected movements cause misclicks, lost reading position, and user frustration.
As a Core Web Vital, CLS affects your Google rankings. But beyond SEO, fixing CLS directly reduces user frustration and can prevent costly misclicks, especially on mobile where ad-related layout shifts might cause accidental ad clicks.
Eksempler fra den virkelige verden
A news website reduced their CLS from 0.42 to 0.05 by reserving space for ad slots and adding dimensions to all images, resulting in 30% longer average reading sessions
An e-commerce product page had a CLS of 0.35 because customer review stars loaded late and pushed the "Add to Cart" button down, causing misclicks; fixing this improved the add-to-cart rate by 8%
A blog fixed their CLS issue caused by web fonts loading late by preloading the font file and using font-display: optional, eliminating text layout shifts entirely
A landing page reserved fixed dimensions for a video embed that was causing content below it to jump, bringing CLS from 0.28 to 0.02
Relaterede termer
Core Web Vitals
Core Web Vitals er tre specifikke målinger defineret af Google, der måler den faktiske brugeroplevelse på websteder: indlæsningsydeevne (LCP), interaktivitet (INP) og visuel stabilitet (CLS).
LCP (Largest Contentful Paint)
LCP måler den tid, det tager for det største synlige indholdselement på en side, såsom et hero-billede eller en overskrift, at blive fuldstændigt gengivet.
Lazy Loading
Lazy loading er en teknik, der udskyder indlæsningen af ikke-kritiske ressourcer, såsom billeder under folden, til de faktisk er nødvendige.
Responsive Design
Responsivt design er en webudviklingstilgang, der giver websteder mulighed for at tilpasse sig og vises korrekt på alle skærmstørrelser, fra skærme til smartphones.
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