The " Content-type: text/html" line contains a so called "HTTP header". This header MUST be followed by a blank line. See also our page Externally defined HTML.

This header tells the remote browser that the response contains an HTML script to be interpreted.

While MS Internet Explorer, in absence of such a header, assumes it by default, Netscape Navigator assumes a text default. Apparently the two products didn't agree on a standard approach. But this is just one of the many examples of disagreement.