This is the VALID HTML code-

<h1 ${mail_script:element.inline_style.generator.title}>
  Issue assigned to your group
</h1>

${mail_script:element.inline_style.generator.empty_string}
<p ${mail_script:element.inline_style.generator.text}>
  Hello ${caller_id.first_name},
</p>
${mail_script:element.inline_style.generator.empty_string}
<p ${mail_script:element.inline_style.generator.text}>
  The Issue, noted below, has been assigned to your group.
</p>
${mail_script:element.inline_style.generator.empty_string}

<h2 ${mail_script:element.inline_style.generator.subtitle}>
  Details
</h2>

<table ${mail_script:element.inline_style.generator.table}>
  <tbody>
    <tr ${mail_script:element.inline_style.generator.tr}>
      <th ${mail_script:element.inline_style.generator.th}>Number</th>
      <td ${mail_script:element.inline_style.generator.td}>${mail_script:element.esc_portal.inc.number.link.styled}</td>
    </tr>
    <tr ${mail_script:element.inline_style.generator.tr}>
      <th ${mail_script:element.inline_style.generator.th}>Caller</th>
      <td ${mail_script:element.inline_style.generator.td}>${caller_id}</td>
    </tr>
    <tr ${mail_script:element.inline_style.generator.tr}>
      <th ${mail_script:element.inline_style.generator.th}>Short Description</th>
      <td ${mail_script:element.inline_style.generator.td}>${short_description}</td>
    </tr>
    <tr ${mail_script:element.inline_style.generator.tr}>
      <th ${mail_script:element.inline_style.generator.th}>Priority</th>
      <td ${mail_script:element.inline_style.generator.td}>${mail_script:priority.color.code} ${priority}</td>
    </tr>
    <tr ${mail_script:element.inline_style.generator.tr}>
      <th ${mail_script:element.inline_style.generator.th}>Assignment Group</th>
      <td ${mail_script:element.inline_style.generator.td}>${assignment_group}</td>
    </tr>
    <tr ${mail_script:element.inline_style.generator.tr}>
      <th ${mail_script:element.inline_style.generator.th}>Opened</th>
      <td ${mail_script:element.inline_style.generator.td}>${opened_at}</td>
    </tr>
    <tr ${mail_script:element.inline_style.generator.tr}>
      <th ${mail_script:element.inline_style.generator.th}>Description</th>
      <td ${mail_script:element.inline_style.generator.td}>${description}</td>
    </tr>
  </tbody>
</table>

${mail_script:element.inline_style.generator.empty_string}
<div ${mail_script:element.inline_style.generator.button_wrapper}>
  ${mail_script:element.esc_portal.go_to_inc.button_link.styled}
</div>

${mail_script:element.inline_style.generator.empty_string}
<p ${mail_script:element.inline_style.generator.text}>
  Please review the Issue and take the appropriate action, you can do so by clicking the number above or going to ${mail_script:element.esc_portal.myincidents.link.styled} in ElementAssist.
</p>
${mail_script:element.inline_style.generator.empty_string}

And this is what it looks like when I put a letter or word into the description field and update (which should in theory do nothing to the formatting, but thanks to whatever 'helper' code SN has...)

<h1 title="">Issue assigned to your group</h1>
<p>${mail_script:element.inline_style.generator.empty_string}</p>
<p>Hello ${caller_id.first_name},</p>
<p>${mail_script:element.inline_style.generator.empty_string}</p>
<p>The Issue, noted below, has been assigned to your group.</p>
<p>${mail_script:element.inline_style.generator.empty_string}</p>
<h2>Details</h2>
<table>
<tbody>
<tr>
<th>Number</th>
<td>${mail_script:element.esc_portal.inc.number.link.styled}</td>
</tr>
<tr>
<th>Caller</th>
<td>${caller_id}</td>
</tr>
<tr>
<th>Short Description</th>
<td>${short_description}</td>
</tr>
<tr>
<th>Priority</th>
<td>${mail_script:priority.color.code} ${priority}</td>
</tr>
<tr>
<th>Assignment Group</th>
<td>${assignment_group}</td>
</tr>
<tr>
<th>Opened</th>
<td>${opened_at}</td>
</tr>
<tr>
<th>Description</th>
<td>${description}</td>
</tr>
</tbody>
</table>
<p>${mail_script:element.inline_style.generator.empty_string}</p>
<div>${mail_script:element.esc_portal.go_to_inc.button_link.styled}</div>
<p>${mail_script:element.inline_style.generator.empty_string}</p>
<p>Please review the Issue and take the appropriate action, you can do so by clicking the number above or going to ${mail_script:element.esc_portal.myincidents.link.styled} in ElementAssist.</p>
<p>${mail_script:element.inline_style.generator.empty_string}</p>

Again this is what the end product looks like with good code-

And after updating anything in the notification-