It comes down to: in the dispatch fase you need a specific amount of fuel, being taxi, trip, cont, alt and final reserve fuel.
The dispatch fase ends when the airplane starts moving under its own power for the purpose of take off.
Inflight the only requirement is to land with at least final reserve fuel! If landing with less than that you have to declare an emergency!!
So to summarize: it is completely legal to sit at a runway and burn through your cont and alt fuel and still take off!!!
Whether that is sensible depends on the situation ( actually had that happen one time in LHR on a flight to AMS with TS in the departure area but severe clear in AMS, decision was taken to continue AMS taking into account all factors but fully aware that if anything happened it would immediately be an emergency).
Above based on EU OPS and notwithstanding company policies that might be more restrictive (I guess if it's in PART A it becomes the regulation).