todoml-after.js

function todoml(string) {
  return  string
  .split('\n')
  .reduce((html, line) => {

    // prepare the line
    line = line.trim()
      // bold
      .replace(
        /(\*\*)([^\*]+)(\*\*)/g,
        (string, open, match, close) => `<strong>${match}</strong>`
      )
      // italic
      .replace(
        /(\*)([^\*]+)(\*)/g,
        (string, open, match, close) => `<em>${match}</em>`
      )
      // underline
      .replace(
        /(_)([\w\s]+)(_)/g,
        (string, open, match, close) => `<u>${match}</u>`
      )
      // strikethrough
      .replace(
        /(~)([^~]+)(~)/g,
        (string, open, match, close) => `<del>${match}</del>`
      )
      // code
      .replace(
        /`([^`]+)`/g,
        (string, match) => `<code>${match}</code>`
      )
      // hyperlink
      .replace(
        /\[([^\]]+)\]\(([^\)]+)\)/g,
        (string, text, url) => `<a href=${url}>${text}</a>`
      )

    // raw HTML
    if (/^<.+\>/.test(line)) {
      html += line
    }

    // paragraph
    if (/^[^#<>`-]/.test(line)) {
      html += `<p>${line}</p>`
    }

    // horizontal rule
    if (/^---/.test(line)) {
      html += `<hr>`
    }

    // heading
    if (/^(#+)\s*/.test(line)) {
      const heading = 'h' + line.match(/^(#+)\s*/)[1].length
      html += `<${heading}>${line.replace(/^#+\s*/, '')}</${heading}>`
    }

    // blockquote
    if (/^(>+)\s*/.test(line)) {
      const level = `class=level-${line.match(/^(>+)\s*/)[1].length}`
      html += `
        <blockquote ${level}>
          ${line.replace(/^(>+)\s*/, '')}
        </blockquote>
      `
    }

    // third-level list
    if (/^-\s+-\s+-\s+/.test(line)) {
      html += `
        <ul>
          <li>
            <ul>
              <li>
                <ul>
                  <li>${line = line.replace(/^-\s+-\s+-\s+/, '')}
                </ul>
            </ul>
        </ul>
      `
    }

    // second-level list
    if (/^-\s+-\s+/.test(line)) {
      html += `
        <ul>
          <li>
            <ul>
              <li>${line = line.replace(/^-\s+-\s+/, '')}
            </ul>
        </ul>
      `
    }

    // list item
    if (/^-\s+[^\[]/.test(line)) {
      html += `
        <ul>
          <li>${line.replace(/^-\s+/, '')}
        </ul>
      `
    }

    // empty checkbox
    if (/^-\s*\[\s+\]/.test(line)) {
      html += `
        <label>
          <ul>
            <li><input type=checkbox>${line.replace(/^-\s*\[\s+\]/, '')}
          </ul>
        </label>
      `
    }

    // checked checkbox
    if (/^-\s*\[[xX]\]/.test(line)) {
      html += `
        <label>
          <ul>
            <li><input type=checkbox checked>${line.replace(/^-\s*\[[xX]\]/, '')}
          </ul>
        </label>
      `
    }

    return html

  }, '')
}

todoml-before.js

function todoML() {

  var script = document.querySelectorAll('script[type="text/todo"]')

  for (var i=0; i<script.length; i++) {

    var todoDoc = ''
    var source = script[i].textContent.split('\n')
    var output = document.createElement('div')

    for (var j=0; j<source.length; j++) {

      var line = source[j].replace(/^\s+/, '')

      // Bold
      line = line.replace(/(\*\*)([^\*]+)(\*\*)/g,
               function(string, open, match, close) {
                 return '<strong>' + match + '</strong>' })

      // Italic
      line = line.replace(/(\*)([^\*]+)(\*)/g,
               function(string, open, match, close) {
                 return '<em>' + match + '</em>' })

      // Underline
      line = line.replace(/(_)([\w\s]+)(_)/g,
               function(string, open, match, close) {
                 return '<u>' + match + '</u>' })

      // Strikethrough
      line = line.replace(/(~)([^~]+)(~)/g,
               function(string, open, match, close) {
                 return '<del>' + match + '</del>' })

      // Code
      line = line.replace(/`([^`]+)`/g,
               function(string, match) {
                 return '<code>' + match + '</code>' })

      // Hyperlink
      line = line.replace(/\[([^\]]+)\]\(([^\)]+)\)/g,
               function(string, text, link) {
                 return '<a href="' + link + '">' + text + '</a>' })

      // Raw HTML Tag
      ;/^<.+\>/.test(line) &&
        (todoDoc += line)

      // Paragraph
      ;/^[^#<>`-]/.test(line) &&
        (todoDoc +=
          '<p>'
          + line
          + '</p>')
      // Heading
      var heading = line.match(/^(#+)\s*/)
      var tag = heading && ('h' + heading[1].length)
      ;heading &&
        (todoDoc +=
          '<' + tag + '>'
          + line.replace(/^#+\s*/, '')
          + '</' + tag + '>')
      // Blockquote
      var blockquote = line.match(/^(>+)\s*/)
      var level = blockquote &&
                    ('class="level-' + blockquote[1].length + '"')
      ;blockquote &&
        (todoDoc +=
          '<blockquote ' + level + '>'
          + line.replace(/^(>+)\s*/, '')
          + '</blockquote>')
      // List item
      ;/^-\s+[^\[]/.test(line) &&
        (todoDoc +=
          '<ul><li>'
          + line.replace(/^- /, '')
          + '</li></ul>')
      // Empty checkbox
      ;/^-\s*\[\s+\]/.test(line) &&
        (todoDoc +=
          '<label><ul><li>'
          + '<input type=checkbox>'
          + line.replace(/^-\s*\[\s+\]/, '')
          + '</li></ul></label>')
      // Checked checkbox
      ;/^-\s*\[[xX]\]/.test(line) &&
      (todoDoc +=
        '<label><ul><li>'
        + '<input type=checkbox checked>'
        + line.replace(/^-\s*\[[xX]\]/, '')
        + '</li></ul></label>')
    }
    output.innerHTML = todoDoc
    script[i].parentNode.insertBefore(output, script[i])
  }
}