Except of course that as the plugin contains <html, it too cannot be saved. Looks like it’s something the webhost will need to sort out.
Actually, it would be a trivial plugin to do that. Just have
$modx->documentOutput = str_replace('< html', '<html', $modx->documentOutput);
and repeat the line for the other tags as required. It wouldn’t be a good idea to just use one line to replace any < followed by a space, since those are often used as arrows or for other purposes in the content (although those really should be expressed by their htmlentities, such as >).
You could just use one line if you wanted to use a regular expression replacement, but those give me a headache.