See URI scheme, BIP 21:
- http://en.bitcoin.it/wiki/BIP_0021
Example:
This is the easiest part, which is fully-documented in BIP 21: URI Scheme. Here a simple example of how you would include it (using my donation address, of course!)
...which looks like: Donate 1.0 BTC to the Armory Project!
Yea but that's just a URI link to open someone's local wallet. Adding a metatag opens the doors to more possibilities and layers on top. Like OP stated browser extensions. In the distant future when bitcoin is widespread the metatag could be scrapped by google's robots and added to results. So if you search for "Brooklyn Cupcakes" you'll receive all the matches, plus they could use that metatag to indicate they accept bitcoin.
Brooklyn Designer Cakes
www.designercakes.net
We offer and makes amazing cakes for everybody, order today with Visa.
Matty's Magnificent Cakes
www.mmcakes.ny BTC
I make all my cakes by hand with natural ingredients. Come down and visit us and try our coffee. Free wifi!
Mastercard Cakes
www.mastercakes.us
We are really old and do things the old way. We only accept cash and mastercard. Newspapers also available for $1.25
In the above example Matty has the bitcoin metatag so he gets a little BTC by his search result.