You have to make sure that the HTML code in the HTML module has:
1. no mismatched/unclosed tags
Did you close the <center> tags with </center>? Btw, center tags are already deprecated (although browsers may still render them properly. You can use vB5's predefined utility CSS class h-align-center to center elements.
Code:
<div class="h-align-center">
content here
</div>
or
Code:
<div style="text-align: center;">
content here
</div>
2. no doctype, <html>, <head>, <body> tags. Otherwise, you will have duplicates as the page already has them and you will get unexpected browser behavior.