docs: def _format_alignment(self, a1, a2):
          """Returns `a1` marked up with HTML spans around characters that are
          also at the same index in `a2`.
  
          :param a1: text sequence from one witness
          :type a1: `str`
          :param a2: text sequence from another witness
          :type a2: `str`
          :rtype: `str`
  
          """
          html = []
          for index, char in enumerate(a1):
              output = self._substitutes.get(char, char)
              if a2[index] == char:
                  html.append('<span class="match">{}</span>'.format(output))
              elif char != '-':
                  html.append(output)
          return ''.join(html)