pre > code (preferred) + buttons

<u1-code>
    <pre>
        <code>
            Shows decoded innerHTML
        </code>
    </pre>
</u1-code>

direct, no tag

<u1-code> Shows decoded innerHTML </u1-code>

foreign element (live! innerHTML)

Change the code

Style Tag (live!)

Script Tag

Textarea

Textarea auto-height with css (field-sizing:content)