static convertSrtCue_(caption) {
    const lines = caption.split(/\n/);

    // detect and skip numeric identifier
    if (lines[0].match(/\d+/)) {
      lines.shift();
    }

    // convert time codes
    lines[0] = lines[0].replace(/,/g, '.');

    const webvttCue = lines.join('\n')
        .replace(/{b}/g, '<b>')
        .replace(/{\/b}/g, '</b>')
        .replace(/{i}/g, '<i>')
        .replace(/{\/i}/g, '</i>')
        .replace(/{u}/g, '<u>')
        .replace(/{\/u}/g, '</u>');

    return webvttCue + '\n\n';
  }
Convert a SRT cue into WebVTT cue

@param {string} caption
@return {string}
@private