sIFR (short for "scalable Inman Flash Replacement") is a technique for rendering a portion of the text on a web page in a font that is not installed on the user's computer. It was originally developed to provide an alternative to the Microsoft Core Fonts, allowing web design to approach the level of font customization present in print design. sIFR accomplishes its custom font rendering by repackaging fonts as Flash files, and then dynamically reconstructing any text that is marked for replacement with scaled copies of each of the letters and symbols.
sIFR presented an improvement over conventional image replacement techniques because it allowed for easy resizing of text and preserved the original text in a hidden section of the site, allowing users to copy and paste content effectively and giving a more level standing to those who access the web with a screen reader. However, sIFR presents notable problems in that it creates very long page loads (any designer worth his salt is aware of the painfully large sizes of Flash files) and places substantial demands on the client computer when rendering the font. It is also rather cumbersome to recompile the fonts into a format that the sIFR engine can manipulate, but this problem is alleviated somewhat by the potential reuse of the files.
In recent times, Cufon has emerged as a competitor to sIFR, boasting higher speeds and simpler configuration options, as well as a lack of reliance on the proprietary Flash plugin. Nonetheless, sIFR remains an extremely popular method for replacing text on websites, and is expected to continue to flourish until the final realization of the @font-face CSS attribute is attained.