From LimeSurvey Manual
CSS class (css_class)
If you want to add special CSS classes to certain questions, you can enter the CSS class name(s) in this box. Make sure you leave an empty space between different class names.
- Any text string with a space between different CSS class names.
expression in this box (New in 3.0.0 ). Remember that the output of the expression will not be updated dynamically.
According to the W3C, CSS class names can contain only the characters [a-zA-Z, and 0-9] and ISO 10646 characters U+00A1 and higher, plus the hyphen (-) and the underscore (_). They cannot start with a digit, or a hyphen followed by a digit. LimeSurvey encodes CSS classes, but it does not fix it totally.