Plone is meant to be useable in all browsers, and a Plone site out of the box should look consistent across all modern browsers, and should be useable on less common browsers (such as Netscape 4.7, cellphone browsers, and non-visual browsers).

However, custom designs applied to Plone sites can be a little more fragile, due to discrepancies across different types and versions of browsers. As of mid-July 2007, browsers likely to account for the vast majority of most sites' traffic (note that this varies with your site's demographics!) are: Internet Explorer 6.x and 7.x (Windows only), Firefox 1.5.x and 2.x (cross-platform), and Safari 2.x (Mac only).