It displays correctly in some browsers, yes, but not everything that speaks HTTP is a browser.  For example, the invalid header makes C#'s WebRequest throw an exception.

I hadn't noticed the 'Content-Type' on the next line of the string output by reset().  That does make things more complicated.

We could put the "Content-Type: text/html" first, but the downside is that it will be output as visible content if a script crashes after the headers have been emitted.

I'm not sure if that's better or worse than emitting an invalid header.