In my case, it happened due to some CSS using Type 1 font (Helvetica).
Internet Explorer changed its font-rendering from IE9 (affects IE10 as well) which does not support the old Type 1 fonts. But still some users manually install fonts (for me, it was Helvetica, tested on Windows 7, IE9 and IE10 both).
Now if you use CSS like:
font-family: Helvetica, Arial, sans-serif;
rather than falling back for Arial, IE9 and IE10 switch to compatibility mode.
More details can be found at: http://bobbyjoneswebdesign.blogspot.com/2011/12/internet-explorer-9-type-1-font-bug.html
As per the mentioned blog post, following approaches would help resolve the problem for the web developers:
- Don't use Helvetica or other non-standard fonts in your CSS
- Use a substitute web font with the CSS @font-face feature to serve up your desired fonts
- Use an online web fonts service like webfonts.fonts.com
- Use Conditional Comments to create IE9 specific stylesheets.