The second Stiefel-Whitney classes of l-adic cohomology

For a proper smooth variety of even dimension over a field of characteristic different from 2 or l, the second Stiefel-Whitney class of the l-adic cohomology and the second Hasse-Witt class of the de Rham cohomology are both defined in the second Galois homology. We state a conjecture on their relation and give several evidences.