What should Microsoft do instead? (12:30 -0800)
Daniel Glazman suggested that those of us criticizing Microsoft's <meta> tag plan should offer an alternative. So here's mine.
Microsoft's main compatibility problem isn't on the Web. The success of Firefox and other browsers more standards-compliant than IE have shown that IE's brokenness isn't needed on the Web. Microsoft's compatibility problem is with content hidden behind corporate firewalls (that is, on Intranets). So what Internet Explorer needs is an Intranet mode, where companies deploying Internet Explorer as both a Web browser and an Intranet browser can configure it to browse their Intranets in compatibility mode without hurting the Web.
How would this work? Internet Explorer could be configured with domain-matching or IP-address-matching rules to select which hosts are "Intranet" hosts. (This configuration would generally be done by those doing the company-wide deployment, and there could be an easy way for users to pick up the configuration in their own installation of Internet Explorer.) Then, for an Intranet host, compatibility would work much like Microsoft just proposed. Compatibility would default to IE7 standards mode or quirks mode, but a <meta> tag or other similar solution could be used to opt in to newer standards support. However, on the Web, standards mode and quirks mode should both continue to evolve, become more standards compliant, and support new features, without any special opt-in. Quirks mode should have as few differences from standards mode as possible: only those that are critical for Web-compatibility.
Microsoft shouldn't hold the progress of the whole open Web hostage to the requirements of Intranets.
