If I want people building mashups with my site, I'll provide an API, HTML is not an API.
If I want Google to see my data, I'll provide it to them when they crawl, more accurately, if I'm ajaxing in data with JavaScript, it's usually explicitly because I don't want crawlers getting to it.
It is not a web developers job to go out of his way to support people who deliberately break their browsers and more often than not contribute very little to the bottom line. Most of us are building apps for people that actually want to use them as intended.