The data we collected for HTTP headers was mostly an afterthought and as such isn't very reliable. Here are some things we noticed, though:
text/html documents without a
charset parameter in the Content-Type
header outnumber those with such a parameter almost by a
factor of two (despite the HTML4 spec saying that UAs must not
assume any default value for the "charset"
parameter
).
Documents with the text/xml MIME type
outnumber documents with the application/xml MIME
type by at least three to one (despite the fact that the former is
discouraged by the XML standards community because of the rules
for how to handle character sets with those MIME types).
There are only twice as many text/plain
documents out there than application/msword documents
(and that doesn't take into account the fact that
text/plain is the default MIME type of some servers
while many application/msword documents will end up
labelled as something else).
The Set-Cookie
header (which is one of the ten most-used headers) is present on
about two orders of magnitude more pages than the Set-Cookie2
header (despite the former being considered insecure).
A pretty significant number of pages include an
X-Pingback header (more than the number of pages with
the Set-Cookie2 header). In fact,
X-Pingback was the 30th most-seen header in our data
sample. The X-Pingback header is part
of Pingback, a blogging technology for tracking responses
similar to Trackback.
There are pages that use the Window-Target
header, and even some that use the Link header
(though we haven't yet checked what for!). There are even some
pages that include the Content-Style-Type
header.