Footnote

As of October 2016 the footnote/tiny print at the bottom of xkcd.com pages reads:

xkcd.com is best viewed with Netscape Navigator 4.0 or below on a Pentium 3±1 emulated
in Javascript on an Apple IIGS at a screen resolution of 1024x1. Please enable your ad blockers, disable high-heat drying,
and remove your device from Airplane Mode and set it to Boat Mode. For security reasons, please leave caps lock on while browsing.

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.

This footnote was added October 4th or 5th, 2016 [1].

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.

BTC means bitcoin. The string of alphanumeric characters is his bitcoin address.

It was removed on September 9th, 2016 [2].