Writing HTML in script tab causes it to render in the run pane errors

[AlexJerabek]

Bug Report

• Host: Excel
• OS: Windows 10
• Environment: Alpha

Expected behavior:

The run pane's syntax error display should not be affected by anything.

Actual behavior:

Having header or button tags inserts headers and buttons into the run pane.

Steps to Reproduce:

1. Use the following code in the "Script" tab.

< section class="setup ms-font-m" >
  <h3>Set up < /h3>
    <button id = "addSampleData" class="ms-Button">
      <span class="ms-Button-label" > Add sample data < /span>
        </button><p>
        < button id = "create-chart" class="ms-Button" >
          <span class="ms-Button-label" > Create chart < /span>
            < /button>
            < /section>

Screenshot:

[Zlatkovsky]

Fixed by #461

[Zlatkovsky]

Looks like it's still reproing with Alex's repro above.
Putting into "polish" milestone to be looked at a bit later.

[Zlatkovsky]