Unlike web safe fonts, web fonts are not pre-installed on the user's system. The fonts are downloaded by the user's browser while rendering the webpage, and then applied to your text. The main drawbacks of using web fonts is it will slow your site's load time. There is also limited support for CSS3 in older browsers which is required to use web fonts. The later limitation can be remedied by using a font stack, similar to the web safe font stacks, but including a web font as the first font of the stack. If a browser is unable to use the web font it will fall back on the web safe fonts in the stack.