Good idea, but I'm afraid that is not generally possible. Consider

  <?php
  ob_start();
  trigger_error(…);
  header('Content-Type: …);
  echo ob_get_clean();

You are correct, however in my opinion, that should be considered as bad practice.
PHP should still output the appropriate format, as reasonably possible. Obviously not all types can be covered, but it can handle them much better than current state.

Also, PHP should at least give an option to make whatever it outputs in the text-only or rich HTML format, in order to override what it has determined by itself.
For example, this can make outputting HTML from cli, perhaps by calling a function like: void set_output_format( int PHP_FORMAT_TEXT | PHP_FORMAT_HTML );
Maybe even automatically setting the Content-Type to text/plain instead of the normal html, if this function is called in cgi mode and set_output_format() is set to PHP_FORMAT_TEXT.

This would be a really useful feature for developers in my opinion, especially when writing a quick script in improper environments.

@cmb, we could also (and I know you'll hate me for saying this), implement it as an ini option to auto detect headers for formatting, thrus disabling it by default to keep BC.

@kalle Why would @cmd hate something like this? I think this would be a great feature for us developers!