Redmond, WA - 29 August 2013
Internet Explorer has long supported EOT web fonts. Beginning with IE9, Internet Explorer supported WOFF fonts and raw OpenType fonts that have their embedding rights bits (set in the OS/2 table) set to “installable”.
Many web font services use the UserAgent string to detect the browser's version and thus determine which font file formats to serve to the browser. When Microsoft released IE10, many sites incorrectly detected the new IE version and served raw OpenType fonts - which IE disregarded due to embedding rights. Once these sites updated their logic to detect IE10 as a successor to IE9, they served the appropriate fonts to the browser.
IE11 is again changing UA string to improve compatibility with other browsers. Once again, some web font services incorrectly detecting the browser version. Web font service providers should verify that their browser detection logic accurately identifies IE 11.
You can find more details about what new UA string is on MSDN: http://msdn.microsoft.com/en-US/library/ie/bg182625(v=vs.85).aspx#uaString
article posted 29 August 2013 and last updated 29 August 2013.