We instrumented Chrome and measured the download times: approximately ~1.1% of font loads are longer than 3s. New timeout will help address that 1.1% by using a system fallback.
As far as the browser is concerned, there is no distinction between icon fonts or any other fonts. If your icon font fails to load within 3s then it'll go ahead and try to paint the page with a fallback -- the results of that will vary based on how your icon font is setup / what glyphs it maps to.
If you want to define own load behavior: use the upcoming Font Load Events API.
