wikijs/server/renderers/modules/markdown-it-underline.mjs

function renderEm (tokens, idx, opts, env, slf) {
  const token = tokens[idx]
  if (token.markup === '_') {
    token.tag = 'u'
  }
  return slf.renderToken(tokens, idx, opts)
}

export default (md) => {
  md.renderer.rules.em_open = renderEm
  md.renderer.rules.em_close = renderEm
}