Yellow-legged Gull features are: 1) outer greater covert pattern is rather uniformly dark, 2) few second generation coverts are present, 3) darkish outer web on inner primaries, although with a rather large pale 'window' on the outer web, perhaps a little too large for michahellis.
Herring Gull features are: 1) argentatus structure, with head shape perhaps not being angular enough for michahellis, 2) tertials having 'oak leaf' pattern with pale notching together with white edge reaching the greater coverts in some of the tertials, 3) fresh plumage without the normal wear associated with early breeding michahellis, 4) inner primaries with dark subterminal tip on inner and outer web, 5) bill size being in the short and slender end for michahellis, 6) tail pattern with narrow tail band and heavy patterned outer feathers is also more argentatus than michahellis like.
Plumage and proportions looks best for argentatus and the michahellis features are actually seen in some Herring Gulls. Not only are the michahellis features within argentatus variation, but michahellis can show notched tertials with white border reaching greater coverts, extensive pale inner webs on inner primaries and it can even show the dark subterminal primary blobs. Thus there is a lot of variation in most of the characters with some overlap between the two species.
The notched 'oak-leaf' tertials as shown by this bird would be very rare in michahellis seen in Denmark and mostly associated with more eastern Yellow-legged Gull such as around Bucharest as seen here, here, here, here, here, here, here, here and here.