- Scan the page and send the contents to Google for analysis, get the response, and serve an ad accordingly (AJAX)
- Build an entire block of HTML from scratch (DOM manipulation – very annoying)
- Fire after the page loads, to ensure all of the content is in place (event handling)
- Override any CSS or other page altering scripts to ensure people can’t be tricked into clicking (CSS hacking!)
- Look and function exactly the same in all browsers
Microsoft has to ensure their ads appear in all browsers, especially Firefox (#2 browser). You can’t sell your services to advertisers if it’s known that your ads break, and possibly misfire, when the wrong browser hits it. That means they’re forced to use a standards compliant implementation to do their ads.
Maybe this is why they appear to be supporting some standards in IE7. Or, maybe they’re just building in the ones they use. Either way, this explains a lot.