lib/jsdom/browser/parser/index.js
function parseFragment(markup, contextElement) {
  const { _parsingMode } = contextElement._ownerDocument;

  let parseAlgorithm;
  if (_parsingMode === "html") {
    parseAlgorithm = htmlParser.parseFragment;
  } else if (_parsingMode === "xml") {
    parseAlgorithm = xmlParser.parseFragment;
  }

  // Note: HTML and XML fragment parsing algorithm already return a document fragments; no need to do steps 3 and 4
  return parseAlgorithm(markup, contextElement);
}
function parseFragment(markup, contextElement) {
  const { _parsingMode } = contextElement._ownerDocument;

  let parseAlgorithm;
  if (_parsingMode === "html") {
    parseAlgorithm = htmlParser.parseFragment;
  } else if (_parsingMode === "xml") {
    parseAlgorithm = xmlParser.parseFragment;
  }

  // Note: HTML and XML fragment parsing algorithm already return a document fragments; no need to do steps 3 and 4
  return parseAlgorithm(markup, contextElement);
}
https://w3c.github.io/DOM-Parsing/#dfn-fragment-parsing-algorithm