Chris Wilson: In Dean’s recent Internet Explorer 8 and Acid2: A Milestone post, he highlighted our responsibility to deliver both interoperability (web pages working well across different browsers) and backwards compatibility (web pages working well across different versions of IE). We need to do both, so that IE8 continues to work with the billions of pages on the web today that already work in IE6 and IE7 but also makes the development of the next billion pages (in an interoperable way) much easier. Continuing Dean’s theme, I’d like to talk about some steps we are taking in IE8 to achieve these goals.

I’ve been on the IE team for over a decade, and I’ve seen us apply the “Don’t Break the Web” rule in six different major versions of IE in different ways. In IE 6, we used the DOCTYPE switch to enable different “modes” of behavior to protect compatibility. When we released IE 6 in 2001, very few pages on the web were in “standards mode” (my team ran a report on the top 200 web sites at the time that reported less than 1%) – few people knew what a DOCTYPE was, and few tools generated them. We used the DOCTYPE switch in IE6 to change the box model to comply with the standards and enable developers to opt-in to the new behavior. We’d already seen so much content written to IE5.x’s non-standard interpretation of the CSS2 spec that we couldn’t change it without causing a slew of problems.