I noticed that a couple of sites have a built in Chrome search feature. If I type stackoverflow.com and then press space in the address bar I get:
Which takes me to this:
How is this done, the major sites like Stackoverflow and github have this feature, but sites that don't implement this default to a regular google search
This line, which is in the HTML of every Stack Overflow page, is responsible:
<link rel="search" type="application/opensearchdescription+xml" title="Stack Overflow" href="/opensearch.xml">
The format of that XML file is documented here: