If you do not see your HTML code after pasting it, it's possible you may be trying to copy and paste HTML by selecting items on a webpage. This will produce unexpected results.
To copy HTML code (see example below), please open your file in a text editor TextEdit. Choose 'Paste as Plain Text'.
For speedier import, you can also drag/drop any HTML file into the "Signature HTML" box.
<table> <p>My signature</p> </table>