counting-css-classes.js

Array.from(document.styleSheets).reduce((acc, cssStyleSheet) => {
  const classNamesCount = Array.from(cssStyleSheet.cssRules).reduce(
    (count, cssRule) => {
      if (cssRule.selectorText && cssRule.selectorText.startsWith('.')) {
        return count + 1;
      }
      return count;
    },
    0
  );
  return acc + classNamesCount;
}, 0);