Most modern browsers including Internet Explorer 11, FireFox and Safari will display emoticons natively within the browser. Chrome however, along with some older browsers currently does not support Emoji and will instead display a square icon in it’s place.
People often use Emoji when tweeting and if you’re using Chrome to view tweets on this website and can’t see emoticons, I would recommend the Chromoji extension. Anyone with older browsers without Emoji support such as Internet Explorer 8 will need to use a more modern browser.
I would expect Chrome to one day support Emoticons natively within the browser and enable them by default in the not too distant future. The use of Emoji across social networks and elsewhere is a growing trend and Google will not want to be left behind.