tumblr offers up an X-Tumblr-Content-Rating header from its webpages. How is that defined precisely? Is it hand-labeled by Tumblr? Do its users apply it?
$ curl -I puppies.tumblr.com 2>&1 | grep Content-Rating
$ curl -I evawomans.tumblr.com 2>&1 | grep Content-Rating
X-Tumblr-Content-Rating: nsfw
$ curl -I porn.tumblr.com 2>&1 | grep Content-Rating
X-Tumblr-Content-Rating: adult
Blog owners can flag their blogs as "NSFW" or as "adult" (at tumblr.com/blog/YOURBLOG/settings):
Flag as NSFW
NSFW blogs may contain occasional nudity or mature/adult oriented content.Flag as adult
Adult blogs contain substantial nudity or mature/adult oriented content.
See Tumblr’s documentation about it: http://www.tumblr.com/docs/en/nsfw
The Tumblr team might flag blogs, too:
If you're not sure if you should flag your blog you can leave it unflagged, but keep in mind that we might flag it later if we see a lot of mature/adult-oriented content.
Before Yahoo acquired Tumblr, blog owners could only flag their blogs as "NSFW" (and there was no documentation about it).