The following tagline appears at the top of the xkcd main page, regardless of which comic is being viewed.
It's a kind of general mission statement for the xkcd comics. No where is there a mention of humor, though it doesn't exclude it. In fact, there are some true downers.
In the early days (until april 2006) the tagline read as:
- A webcomic of romance and geek humor. Updated on time every M/W/F since 1779.
(Old versions of the site can be viewed here.)