Span gets removed or changed when pasting text.

[Jakub Ś]

Use below code in contenteditable element or paste it into editor:

<table border="0" cellpadding="0" cellspacing="0" style="width:100%">
  <tbody>
    <tr>
      <td align="center">
        Plain text pasted here
        <br>
        <span style="font-size:8pt;">
          PDF, 378 KB
        </span>
      </td>
    </tr>
  </tbody>
</table>

Now copy this Chinese text 字讀音,臺灣教育部異體字字典標同 (can also be done with european text), select PDF, 378 KB and paste the chinese text.

In Firefox span gets removed in editor and native contenteditable.

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In Blink and IE:
In editor:

<td>Plain text pasted here<br><span style="font-size:8pt"></span>字讀音,臺灣教育部異體字字典標同<span style="font-size:8pt"> </span></td>

In native contenteditable:

<span style="text-align: start;">字讀音,臺灣教育部異體字字典標同</span>

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I don't think we can do anything about Firefox but perhaps there is room for improvement for Blink and IE.