org-html-tag-class-prefix

org-html-tag-class-prefix is a variable defined in `ox-html.el'.
Its value is

""

Documentation:
Prefix to class names for TODO keywords.
Each tag gets a class given by the tag itself, with this prefix.
The default prefix is empty because it is nice to just use the keyword
as a class name. But if you get into conflicts with other, existing
CSS classes, then this prefix can be very useful.

You can customize this variable.