Use puppeter to automatically derive a website layout stability score using the new layout instability API (see https://web.dev/layout-instability-api). Currently the feature is still behind experimental feature flag, which might not always work right depending on the Chrome you test with.

layout_stability.js

const puppeteer = require('puppeteer');
const devices = require('puppeteer/DeviceDescriptors');

const Good3G = {
    'offline': false,
    'downloadThroughput': 1.5 * 1024 * 1024 / 8,
    'uploadThroughput': 750 * 1024 / 8,
    'latency': 40
  };
  
const phone = devices['Nexus 5X'];

async function run() {
	// I can't get the experimental flag working on chromium, so let's start system chrome
    const browser = await puppeteer.launch({headless: true,
		                                    executablePath: '/Applications/Google Chrome.app/Contents/MacOS/Google Chrome',
											args:["--enable-experimental-web-platform-features"]});
	
	// whenever the feature is out of experimental we can use headless chromium as usual
	//const browser = await puppeteer.launch({headless:true, args:["--enable-feature=ExperimentalWebPlatformFeatures", "--enable-experimental-web-platform-features"]});
	
	
    let page = await browser.newPage();
    const client = await page.target().createCDPSession();
    await client.send('Network.enable');
    await client.send('Network.emulateNetworkConditions', Good3G);
    await client.send('Emulation.setCPUThrottlingRate', { rate: 4 });
	await page.emulate(phone);
	await page.goto('https://www.spiegel.de');
	const score = await page.evaluate(async function() {
		let score = 0;
		var events = window.performance.getEntriesByType('layoutJank');
		for(var i = 0; i < events.length; i++) {
			score += events[i].fraction;
		}
		return score;
	});
	console.log("Your layout stability score is: " + score + " (higher value is worse. See https://web.dev/layout-instability-api for details.)");
		
    await page.close();
    await browser.close();
}

run();