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    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.

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    Help support the work that goes into creating and maintaining my projects and buy me a coffee via Ko-fi or sponsor me on GitHub Sponsors.

    Getting Started


    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 --device mobile --output layout-shift.gif

    This will generate an animated layout-shift.gif of showing the regions of Cumulative Layout Shift on the viewport of a simulated mobile device.


    Usage: layout-shift-gif --url <url>
          --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"]
      layout-shift-gif --url --device mobile --outpu
      t layout-shift.gif


    Basic Examples

    Simulated Mobile Device

    layout-shift-gif --url --device mobile

    Simulated Desktop Device

    layout-shift-gif --url --device desktop

    Advanced Examples

    Supplying Cookies

    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 --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 --type option.

    layout-shift-gif --url --type old

    Device Simulation

    The tool is able to check both a desktop and a mobile viewport.

    • The desktop viewport is a standard 1920x1080 resolution.

    • The mobile viewport is the Nexus 5X profile from Lighthouse.

    Both the CPU and the network are throttled to simulate a good 3G network.

    Hosted Version

    If you don't wish to use the CLI version there is also a free hosted version running in a cloud function available on


    An outline of how to interpret the GIF output from this tool.

    Page Screenshot

    The screenshot of the page is taken after all the page elements have shifted and the CLS has been calculated.

    Border Style

    The border style of an outlined element represents the start and end positions of the elements shift.

    • A dashed border indicates the element's starting position.

    • A solid border indicates the element's end position.

    Border Colour

    The border colour of an outlined element represents the CLS score of that element against the overall page thresholds for CLS outlined by Google.

    Defining the Core Web Vitals metrics thresholds

    🟢 Good 0.1

    🟠 Needs Improvement

    🔴 Poor > 0.25

    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.

    Corner Metric

    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.

    Corner Colour

    The colour of the top left corner represents where the pages overall CLS score fits within the thresholds for CLS outlined by Google.

    Defining the Core Web Vitals metrics thresholds

    🟢 Good 0.1

    🟠 Needs Improvement

    🔴 Poor > 0.25


    If you find this project useful please considering sponsoring me on GitHub Sponsors and help support the work that goes into creating and maintaining my projects.


    Sponsors are able to remove the project support message from all my projects, as well as access other additional perks.


    Chris Johnson - - @defaced


    npm i layout-shift-gif

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