If you embedded web fonts in the past years (e.g. by copying CSS from a Font Squirrel @font-face kit Show archive.org snapshot ), that CSS won't work in Internet Explorer 9.

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You can fix it by turning these styles...

@font-face
  font-family: 'MyFont'
  src: url('myfont.eot')
  src: local("☺"), ('myfont.ttf') format("truetype")
  font-weight: normal
  font-style: normal

... into these:

@font-face
  font-family: 'MyFont'
  src: url('myfont.eot')
  src: local("☺"), url('myfont.eot?#iefix') format('embedded-opentype'), url('myfont.ttf') format("truetype")
  font-weight: normal
  font-style: normal

Font Squirrel's @font-face kits come also come with this updated CSS now.