Success Criterion 2.4.7. Focus Order

Hi, all.

I'm reading the Understanding SC 2.4.7 about Focus Order, and I'm in 
doubt about what is the meaning of "having a mode of operation that...".

For instance, if I have a hidden "skip to contents" link (via css 
off-left), when a user tabs to this link the focus will not be visible; 
but if the user disables CSS, in this "mode" the link will be visible 
and also the focus. My question is: would this be considered valid? 
Should we show all the links to comply with this criterion? Are 
css-hidden links forbidden?

Thanks in advance,
Ramón.