How did french artists jacques-louis david and antoine-jean gros incorporate ideas of grand manner painting into their depictions of contemporary events?

Jacques-Louis David and Antoine-Jean Gros did not strive for realist ideals, rather an idealized depiction of an event (Le Serment de Jaume de paume, or The Tennis Court Oath, 1791) constructed using ideal proportions and symmetry based on an idealized aesthetic style derived from the classical art period.

Generally spontaneous human combustion is something that still bewilders some, human combustion happens very rarely and doesn't affect the surrounding areas, so unless you want to set your self on fire, human combustion will remain spontaneous and very rare.