Internet Explorer 8 is the first version of IE to pass the Acid2 test, and the last of the major browsers to do so (In the later Acid3 Test, it only scores 24/100.). According to Microsoft, security, ease of use, and improvements in RSS, CSS, and Ajax support were its priorities for IE8.
Internet Explorer 8 is the last version of Internet Explorer to run on Windows Server 2003 and Windows XP; the following version, Internet Explorer 9, works only on Windows Vista and later. Support for Internet Explorer 8 is bound to the lifecycle of the Windows version it is installed on as it is considered an OS component, thus it is unsupported on Windows XP due to the end of extended support for the latter in April 2014. Effective January 12, 2016, Internet Explorer 8 is no longer supported on any client or server version of Windows, due to new policies specifying that only the newest version of IE available for a supported version of Windows will be supported. However several Windows Embedded versions will remain supported until their respective EOL, unless otherwise specified.
IE8 development started in at least March 2006. In February 2008, Microsoft sent out private invitations for IE8 Beta 1, and on March 5, 2008, released Beta 1 to the general public, although with a focus on web developers. The release launched with a Windows Internet Explorer 8 Readiness Toolkit website promoting IE8 white papers, related software tools, and new features in addition to download links to the Beta. Microsoft Developer Network (MSDN) added new sections detailing new IE8 technology. Major press focused on a controversy about Version Targeting, and two new features then called WebSlice and Activities. The readiness toolkit was promoted as something "developers can exploit to make Internet Explorer 8 'light up'."
On August 27, 2008, Microsoft made IE8 Beta 2 generally available. PC World noted various Beta 2 features such as InPrivate mode, tab isolation and color-coding, and improved standards and compatibility compared to Internet Explorer 7. Two name changes included Activities to Accelerators, and the IE7 Phishing filter renamed Safety Filter in the first Beta to SmartScreen, both accompanied by incremental technical changes as well. By August 2008, the new feature called InPrivate had taken the spotlight.
The first non-beta version was released on March 19, 2009.