[9] Internet Explorer 9 also supports the HTML media tags

[11] The Internet Explorer team focused on improving support and performance for HTML5, CSS3, SVG, XHTML, JavaScript, hardware acceleration, and the user interface featuring agility and "a clean new design".

The third Internet Explorer 9 Platform Preview also includes support for audio, video, and canvas tags, and WOFF.

On September 15, 2010, the Internet Explorer 9 Public Beta was released alongside Platform Preview 5, featuring a new user interface.

The sixth Internet Explorer 9 Platform Preview was released on October 28, 2010, and includes support for CSS 2D transforms and HTML semantic elements.

[18] These previews were not full builds of Internet Explorer 9, as they were for testing the latest version of the MSHTML (Trident) browser engine.

Moreover, KB2452648 resolves the in-built feedback issue with Internet Explorer 9 and the latest version of Windows Live Sign-in Assistant.

[24][25] The final version of Internet Explorer 9 was publicly released during the South by Southwest (SXSW) Interactive conference in Austin, Texas, on March 14, 2011.

However, IE9 (via Windows Server 2008) is eligible for the paid ESU (Extended Security Updates) and Premium Assurance programs.

Compiling in the background enables users to keep interacting with webpages while Internet Explorer 9 generates even faster code.

[49] The engine significantly improves support for ECMA-262: ECMAScript Language Specification standard, including features new to the recently finalized Fifth Edition of ECMA-262 (often abbreviated ES5).

The 64-bit version of Internet Explorer 9, which is not the default browser even on 64-bit systems, does not have the JIT compiler[14][51] and performs up to 4 times slower.

Internet Explorer 8's InPrivate Filtering blocked third-party content using an XML list which had to be imported or automatically built a list by observing third-party servers that users kept interacting with as they browsed the web, and once a server showed up more than a set number of times, InPrivate Filtering would block future connections to it[69] Internet Explorer 9 supports two methods of tracking protection.

The primary method is through the use of Tracking Protection Lists (TPL)[70] which are now supplied by internet privacy-related organizations or companies.

At the moment following this header is a voluntary code of conduct but this method could in future be enforced by government legislation.

This helps ensure that structured exception handling cannot be used as an exploit vector, even when running outdated browser add-ons that have not been recompiled to take advantage of SafeSEH.

[72] Internet Explorer 8 used SmartScreen technology, which, according to Microsoft, was successful against phishing or other malicious sites and in blocking of socially engineered malware.

[76] By comparison, the same research showed that Chrome 6, Firefox 3.6 and Safari 5, which all rely on Google's Safe Browsing Service, scored 6%, 19% and 11%, respectively.

[80][81] Due to technical improvements of the browser, the Internet Explorer developer team decided to change the user agent (UA) string.

The Mozilla/4.0 token was changed to Mozilla/5.0 to match the user agent strings of other recent browsers and to indicate that Internet Explorer 9 is more interoperable than previous versions.

The extended string is still available to websites via the browser's .userAgent property, and is sent when a web page is displayed in Compatibility View mode.

Not loading BHOs or toolbars improves startup time, but limits the ability of developers to augment the user experience through these extensibility mechanisms.

He felt it was "fast and polished", a "remarkable improvement" over version 8, noting "superb" development tools and "real and significant" support for HTML5, though "not as comprehensive as the company's publicity implies."

Having reached release candidate status eleven months after it was originally announced at the March 2010 MIX conference, "Microsoft's development process is too slow."

"[88] Computing observed that "the feature set has piled up" since development began, with recent changes including "a completely rejigged JavaScript engine, and far better web standards support."

It reported that Internet Explorer 9 RC ranked above Firefox, slightly above Safari, and below Chrome and Opera on Futuremark's Peacekeeper browser benchmark.

[89] Michael Muchmore's first impressions in PC Magazine were broadly positive, praising features of the InPrivate mode (which "I'm surprised other browser makers haven't included") and concluding that Internet Explorer 9's tracking protection was "more flexible and comprehensive" than Mozilla's.

Bottom line, I really don't think that JavaScript performance is an issue any more, and certainly in real-world testing it's hard to see a difference between the browsers.