icons for web page in location bar of browser-technique?

[Joan K. Graham]

I believe that some time ago there was a discussion on this list
concerning why an icon appeared before the URL in the location bar,
after you accessed certain web sites, while only a browser icon would
appear for other sites.

A colleague asked me yesterday why our site's URL didn't produce this. I
told him I thought it was probably code written into the page, and I
wished I had paid attention, if I did see postings on this topic. Does
anyone know how it's done?
TIA.
Joan Graham
Stark County District Library