Do you want people to write HTML? Please say no because allowing that is painful. Replace
<?php include "comments.html"; ?>with
<?php readfile("comments.html"); ?>and
fwrite($handle,"<b>" . $name . "</b>:<br/>" . $content . "<br/><br>");with
fwrite($handle,"<b>" . htmlspecialchars($name) . "</b>:<br/>" . htmlspecialchars($content) . "<br/><br>");Now go into your comments.html and make sure there isn't anything bad in it.
With that taken care of, find and fix the problem with your starting <head> tag. I expect the browser was able to recover gracefully from it, though, as you would have noticed otherwise.
If you need help with the appearance of your site then we need to see the HTML (your PHP isn't complicated so that's okay) and the CSS. Preferably with a link to the site, but don't provide a link until you do the comment thing I said above. Go to the full post