Layout Shift GIF Generator - CLI

https://defaced.dev/tools/layout-shift-gif-generator/

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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Getting Started

Installation

The Layout Shift GIF Generator command line tool can be installed directly from NPM.

npm install -g layout-shift-gif

Usage

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 layout-shift.gif of https://blacklivesmatter.com/ showing the regions of Cumulative Layout Shift on the viewport of a simulated mobile device.

Options

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

Basic Examples

Simulated Mobile Device

layout-shift-gif --url https://blacklivesmatter.com/ --device mobile

Simulated Desktop Device

layout-shift-gif --url https://blacklivesmatter.com/ --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 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 --type option.

layout-shift-gif --url https://blacklivesmatter.com/ --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 defaced.dev

Output

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.

🟢 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.