var parseErrorHandler = parseCss.config.parseErrorHandler = function (index, token) {
  parseErrorHandler.errors.push(
    `Parser error at index ${index}, token ${token} on css: ${cssText}`;
  );
};

parseErrorHandler.errors = [];
var cssText = /* input css string */;
var tokens = parseCss.tokenize(cssText);
// do something with parseErrorHandler.errors

const {Tokenizer: CssTokenizer} = parseCss;

const tokenizerErrorHandler = function (tokenizer, input, index, token) {
  tokenizer.errors = tokenizer.errors || [];
  tokenizer.errors.push(
    `Parser error at index ${index}, token ${token} on css: ${input}`
  );
};

var tokenizer = new CssTokenizer({errorHandler: tokenizerErrorHandler});
var tokens = tokenizer.run(/* input css string */);
/* do something with tokenizer.errors */

const {Parser: CssParser} = parseCss;

const parserErrorHandler = function (parser, input, index, token) {
  parser.errors = parser.errors || [];
  parser.errors.push(
    `Parser error at token ${index}: ${token}`
  );
};

var parser = new CssParser({errorHandler: parserErrorHandler});
var result = parser.parseAStylesheet(/* input css string */);
/* do something with parser.errors */