Yahoo is joining efforts by Microsoft, Google, and legions of developers to wean the Web of Internet Explorer 6, reducing its support for how well its Web programming tools works with 2001-era browser.

Yahoo announced the IE6 demotion last week for its Yahoo User Interface (YUI) tools, an open-source project that supplies Web developers with code modules they can use to build sophisticated sites. Yahoo, naturally, is among its users.

IE6, introduced in 2001, lacks support for many Web standards--many old ones as well as a host of important new ones--and is feeble at processing Web-based JavaScript programs essential to the new era of dynamic, rich sites. It's also more vulnerable to security threats. Although IE6 usage is gradually waning, it's still common, in part because it's built into Windows XP and hard to dislodge from corporate computing environments.

Dealing a blow to the browser, YUI developers Eric Miraglia and Matt Sweeney in a blog post said Yahoo expects to "discontinue A-grade for Internet Explorer 6, moving it to C-Grade" in the first quarter of 2011 so there's more time to focus on mobile browsers and other important new areas.

That doesn't mean that IE6 users will see broken Yahoo Web sites, starting in a few months, just that the YUI tools will only support basic functions. Here's how Yahoo puts it:

We are forecasting the transition of Internet Explorer 6 from A-grade to C-grade in the next GBS [graded browser support] update. The calculus here is simple: The proliferation of devices and browsers on the leading edge (including mobile) requires an increase in testing and attention. That testing and attention should come from shifting resources away from the trailing edge. By moving IE6 to the C-grade, we ensure a consistent baseline experience for those users while freeing up cycles to invest in richer experiences for millions of users coming to the Internet today on modern, capable browsers.

Microsoft has been trying to move people off IE6 for months, including among other things a campaign likening IE6 to spoiled milk. Google, whose business relies on browsers and suffers from the problems of old ones, is phasing out IE6 support for many of its sites.

Up and coming on the Yahoo support list are mobile browsers. In the current quarter's update of browser support, the built-in for Apple's iOS 3.x and 4.x and for Google's Android 2.2 attained A-level support. That should ease programming chores for those using YUI to reach the mobile realm.

When Microsoft releases the final version of IE9 and Mozilla releases the final version of Firefox 4, those browsers also will get A-level support, Yahoo said.