As of October 2016 the footnote/tiny print at the bottom of xkcd.com pages reads:
The text gives questionable advice on how to view xkcd.com. Using a discontinued browser on an Apple computer released in 1986 with a obscure screen resolution there basially just is a horizonal line.
at a screen resolution of 1024x1. If the dimensions given are in pixels, as they usually are, then the recommended display setting would only show one horizontal line. 1024 pixels is wider than the maximum supported display width of the Apple IIGS. Please enable your ad blockers, Many sites ask users to disable ad blockers, either so the owner can get ad revenue, or because blockers sometimes inadvertently block other parts of the page. disable high-heat drying, This appears to be referring to clothes dryer heat settings, which are irrelevant to websites. Some clothing is damaged if dried on with high heat. and remove your device from Airplane Mode and set it to Boat Mode. Many portable devices, especially cell phones and tablets, have an "Airplane Mode," which disables the wireless radios to avoid potentially interfering with an aircraft's operation while flying in it. "Boat Mode" is fictional. (Though it might be nice to have a boat mode that turns the phone off if dropped, to reduce water damage.) For security reasons, please leave caps lock on while browsing. Having caps lock on would not improve security. It may reduce your security if it prevents you from using lower-case letters in passwords.
This footnote was added October 4th or 5th, 2016 .
 Old footnote
Previously the footnote was:
BTC 1FhCLQK2ZXtCUQDtG98p6fVH7S6mxAsEey We did not invent the algorithm. The algorithm consistently finds Jesus. The algorithm killed Jeeves. The algorithm is banned in China. The algorithm is from Jersey. The algorithm constantly finds Jesus.
And the following one added by Randall:
This is not the algorithm. This is close.
It was added by Randall in April 2007, according to his Blog as a response to random billboards appearing in the New York, Los Angeles and San Francisco areas. It turned out these were a viral marketing campaign by the ask(jeeves) search engine to drive publicity around their new search algorithm. The campaign is long over, but Randall kept the text there (apparently) as a self referential advertising campaign. Specifically, people who find the small text will use a search engine to see what it means and the search engine will likely lead them back to xkcd — where they saw the text initially.
It was removed on September 9th, 2016 .