Adding the registry key to the same path in HKCU instead of HKLM will also work - this is useful as writing to HKLM requires admin privileges where as HKCU does not.
And in there add 'myApplicationName.exe' with value '9000' to force the WebBrowser control to display pages in IE9 mode. Though there are other values you can use too too, note that these docs aren't entirely accurate as it does not seem possible to get a page to render in IE 8 mode whatever value you use.
Be careful, if you are running 32-bit applications (that call the 32-bit MSIE) on a 64-bit Windows, the registry entry should be added to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Internet Explorer\MAIN\FeatureControl\FEATURE_BROWSER_EMULATION instead.
I tried the meta tag but it would not work. I was getting an error message stating "HTML1115: X-UA-Compatible META tag (IE=9) ignored because document mode is already finalized.", which lead me to the webpage evolpin.wordpress.com/2011/02/25/. The solution was then to ensure that the meta tag was the first element inside the <head> block.
I was rendering html through xsl/xslt and the regedit didn't worked for some reason... The meta did the trick. Thanks!
If you want to take advantage of new IE9 features, you should add the meta tag <meta http-equiv="X-UA-Compatible" content="IE=9" > inside the <head> tag of your HTML page.
WebBrowser control will use whatever version of IE you have installed, but for compatibility reasons it will render pages in IE7 Standards mode by default.