Used to optimize the loading of Google Fonts. 'Flash of invisible text': load fonts in a standard way at the beginning of a HTML page - most browsers do not display text until the font is loaded. 'Flash of unstyled text': load fonts asynchronouslty and switch from default font to the loaded one when ready. 'WebFont Loader': load fonts asynchronously using the webfont.js library. 'None': disable optimization.