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npm install webdriverio wdio-screenshot wdio-mocha-framework
./node_modules/.bin/wdio wdio.config.js

test.js

suite('suite', function() {
  test('test', function() {
    browser.url('https://many-worlds.gomix.me/mathquill/mathquill/branch/ci.cleanup/test/visual.html')
    .saveDocumentScreenshot('edge_screenshot.png');
  });
});

wdio.config.js

exports.config = {
  user: 'mathquill',
  key: 'xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx',
  specs: [ 'test.js' ],
  capabilities: [{ browserName: 'MicrosoftEdge', platform: 'Windows 10' }],
  plugins: { 'wdio-screenshot': {} },
  framework: 'mocha',
  mochaOpts: { ui: 'tdd' }
};