Remove the _white-space: pre;_ from your style.

Then the empty lines that are in the source are missing in the output. Looks like the Show HN avoids this by not having empty lines in text.

Upon closer inspection, I now see that the Show HN also has additional empty lines in the rendered view.

End of the source:

    </body>
    </html>

Rendered in the browser as:

    </body>
    
    </html>

I get that extra spacing by using:

    margin:1.5em 0

When there was no margin the text was quite ugly and hard to read. Then tried margin 1em and it was better. Pushing it out to 1.5em really did the trick.