I get the following in IE 9 when visting a page (here the login page)
SCRIPT16385: Not implemented SfdcCore.js, line 71 character 160 SCRIPT5009: "Sfdc" is undefinied VFState.js, line 1 character 1 SCRIPT5009: "Sfdc" is undefinied
The affect of this is an error in the console, the login does not work and also the login validation is not working. I have to switch to the compatibility mode. Then it is working. If you don't know of a fix how can I enable the compatibility mode? Placing a meta tag into the header does not work, because it is rendered too late (IE 9 reports HTML1115 error).