What is "evttag=" used for?

[JDatSB]

I see evttag= used on realtor.com, what looks to be for click tracking purposes.

Does anyone know if this is an official standard or something they made up?

[CoreyNorthcutt]

Well, it's certainly not an SEO thing.

In fact, it's not even a valid HTML thing; here are all of the craziest, acceptable attributes for a DIV in HTML5:

http://dev.w3.org/html5/spec/single-page.html#the-div-element

It among many, many other things, is breaking realtor.com's code through the validator:

http://validator.w3.org/check?uri=http%3A%2F%2Fwww.realtor.com&charset=(detect+automatically)&doctype=Inline&group=0

In my experience, this one of the easiest ways to assure that Google can't crawl your site well, or put confidence in your user experience.   Other crazy new standards, like schema.org do get pretty creative with structured data, but you'll notice that those use itemscope/itemprop/itemtype ....  which do still pass the W3 validator:

http://schema.org/docs/full.html

In addition, there's no reference to an outside standard, even one that's totally foreign to what's generally accepted web development (similar to say, Facebook+OpenGraph, which is sometimes used, like this):

xmlns="http://www.w3.org/1999/xhtml" xmlns:fb="http://www.facebook.com/2008/fbml" xmlns:og="http://opengraphprotocol.org/schema/">

I award realtor.com's web developers 0 points, and may the flying spaghetti monster have mercy on their SEO's.

To put that another way:  don't copy this.