Brion's email reminds me that we sometimes use an icon resembling
Adobe's A to identify PDF, which is very ugly for an open standard.
FSFE's initiative welcomes design help to make their icons better, their
maintainer told me:
http://pdfreaders.org/graphics.en.html
Also, the Internet Archive brought up a proposal to "Create a visual
format/style to include an archived link next to an external link".
https://www.mediawiki.org/wiki/Archived_Pages
Nemo