I do this on some forms and pages. here is a copy to clipboard JavaScript function with it. You can work out what HTML version you want to use. but this is one way.

my $Some_HTML_Content = '';
# we can just convert the arrows for any textarea so we don't brake ou
+t of
# our textarea below.
$Some_HTML_Content =~ s/<\/textarea>/&#60;\/textarea&#62;/g;
print <<"HTML";
<textarea id="html" name="html">$Some_HTML_Content</textarea>
<input type="button" value="Copy To Clipboard" onclick="copy_to_clipbo
+ard('html');">

<!-- Copy To Clipboard -->
<script>
function copy_to_clipboard(id)
{
    document.getElementById('html').select();
    document.execCommand('copy');
}
</script>
HTML
[download]

Update: Or another way I have done it is with JavaScript regex to convert all the HTML entities when you copy to clipboard.

Update2: or you can just wright the file content as Content-Type: text/plain and display it in an Iframe or JavaScript.