we have to namespace CSS classes correctly, otherwise this will collide with any other random CSS dependency (e.g. markdown, syntax highlighter, etc) which might choose to define a generic class like has_prefix. In this instance, I assume it should be something like mx_CreateGroupDialog_input_hasPrefix.

....however, it's very very unclear and confusing as to what the difference is between mx_CreateGroupDialog_input_prefix and mx_CreateGroupDialog_input.has_prefix here, and so i suspect you need a better name anyway.

ah, I see what you're saying

let's go with
.mx_CreateGroupDialog_prefix and .mx_CreateGroupDialog_input_hasPrefixAndSuffix

also, i would avoid the .foo.bar.baz selector format - we don't use it in many other places, and instead do the (different) .foo .bar, .foo .baz selector to say "select classes with name bar or baz which are within a parent class called foo". (Unless there's a performance improvement I've forgotten about explicitly having .foo.bar.baz being "select classes which have names foo, bar & baz"? I may also be a bit historically traumatised by .foo.bar.baz not working reliably on IE4 ;P)