Uses the Puppeteer node module to extract the coverage CSS from Chrome, for a specific URL.

coverageCSS.js

const puppeteer = require('puppeteer');

// Include to be able to export files with node
const fs = require('fs');

// This prepares the filenames for the coverage .css files 
function exportCSSFileName(url) {
    
    // Remove https:// from the beginning of the string
    // Remove .css* from the end of the string
    var newFileName = url.substring(8, url.search("\\.css"));
    
    // Replace all forward slashes with hyphens
    newFileName = newFileName.replace(/\//g, "-");
    
    // Return filename with .css concatenated at the end    
    return newFileName+".css";
}

(async () => {
    const browser = await puppeteer.launch();
    const page = await browser.newPage();

    // Begin collecting CSS coverage data
    await Promise.all([
        page.coverage.startCSSCoverage()
    ]);

    // Extract coverage CSS from this URL
    await page.goto('https://google.com');
  
    // Stop collection and retrieve the coverage iterator
    const cssCoverage = await Promise.all([
        page.coverage.stopCSSCoverage(),
    ]);

    // Investigate CSS Coverage and Extract Used CSS
    const css_coverage = [...cssCoverage];
    
    for (const entry of css_coverage[0]) {
                          
        // Check if the css asset contains '.css' in the URL, if it does then export the coverage CSS
        // Useful for filtering out inline styles in the web page
        if ( entry.url.search("\\.css") !== -1 ) {
            
            // Init covered_css
            let covered_css = "/* Generated by Superfein CSS Coverage Tool \n" + 
                          "   *************************************  */ \n\n";

            for (const range of entry.ranges) {
                
                // Iterate through the .json ranges
                covered_css += entry.text.slice(range.start, range.end) + "\n";
            } 
            
            // Create and export each coverage CSS file
            fs.writeFile("./css_assets/"+exportCSSFileName(entry.url), covered_css, function(err) {
                if(err) {
                    return console.log(err);
                }
                console.log("The file "+exportCSSFileName(entry.url)+" was saved!");
            });            
             
        }     
    } 

    await browser.close();
})();