Layout Shift GIF Generator - CLI
Visualise the Core Web Vitals metric Cumulative Layout Shift (CLS) with a simple GIF.
Lighthouse is a great tool for identifying your overall Cumulative Layout Shift (CLS) score, but it's not so great for quickly visualising what's actually shifting on a page. The Layout Shift GIF Generator allows you to quickly identify which elements are moving around in the viewport.
The Layout Shift GIF Generator command line tool can be installed directly from NPM.
npm install -g layout-shift-gif
Once installed the tool can be used as per the following example.
layout-shift-gif --url https://blacklivesmatter.com/ --device mobile --output layout-shift.gif
This will generate an animated
https://blacklivesmatter.com/ showing the regions of Cumulative Layout Shift on the viewport of a simulated
Usage: layout-shift-gif --url <url> Options: --help Show help [boolean] --version Show version number [boolean] -u, --url Website URL [string] [required] -d, --device Device type [string] [default: "mobile"] -w, --width Device viewport width [number] -h, --height Device viewport height [number] -c, --cookies Cookie filename [string] -o, --output Output filename [string] [default: "layout-shift.gif"] -t, --type CLS calculation method [string] [default: "new"] Examples: layout-shift-gif --url https://blacklivesmatter.com/ --device mobile --outpu t layout-shift.gif
Simulated Mobile Device
layout-shift-gif --url https://blacklivesmatter.com/ --device mobile
Simulated Desktop Device
layout-shift-gif --url https://blacklivesmatter.com/ --device desktop
You can supply a cookie file in the Puppeteer JSON format allowing you to bypass cookie notices, or interstitals.
I recommend using the Export cookie JSON file for Puppeteer Chrome extension to export your cookies in the correct format.
layout-shift-gif --url https://blacklivesmatter.com/ --cookies cookies.json
Original CLS Calculation Method
In June 2021 Google changed how they calculate the CLS metric. Layout Shift GIF Generator defaults to the newest method, but if you require the old method it can still be access viable the
layout-shift-gif --url https://blacklivesmatter.com/ --type old
The tool is able to check both a desktop and a mobile viewport.
desktopviewport is a standard 1920x1080 resolution.
mobileviewport is the Nexus 5X profile from Lighthouse.
Both the CPU and the network are throttled to simulate a
good 3G network.
If you don't wish to use the CLI version there is also a free hosted version running in a cloud function available on defaced.dev
An outline of how to interpret the GIF output from this tool.
The screenshot of the page is taken after all the page elements have shifted and the CLS has been calculated.
The border style of an outlined element represents the start and end positions of the elements shift.
dashedborder indicates the element's starting position.
solidborder indicates the element's end position.
The border colour of an outlined element represents the CLS score of that element against the overall page thresholds for CLS outlined by Google.
This means that if you see a shifting element with a red or orange outline then this element alone is contributing significantly to a negative CLS score for that page.
The metric in the top left corner is the overall CLS score for that page and is the score you'll see in Lighthouse or Pagespeed Insights.
The colour of the top left corner represents where the pages overall CLS score fits within the thresholds for CLS outlined by Google.
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