JSmol (non-Java) version works best in the Chrome browser, and works well in the Firefox, Safari and Opera browsers. Performance in Internet Explorer is
poor; it is advisable to either use another browser or choose the Java modality.
Chrome, Firefox and others cannot run Java any more, so your only choice is the JSmol version (which will be automatically selected by default).
Tablets and smartphones (both Android and iOS-based) are usually not capable of running Java, so your only choice is the JSmol version (which will be automatically selected for those devices).
Help on how to install and enable Java in your browser, and other related information: Installing and enabling Java - a document in the Proteopedia website (main author: Eric Martz)