Note: Minor spoilers.
Similarities I noticed between Whisper of the Heart (1995) and Grave of the Fireflies (1988):
Both films feature songs that deal with the concept of home.
“Take Me Home, Country Roads” is about returning to the place where you belong.
“Home Sweet Home” is about the desire to return home.
Involvement in World War II is a plot point in both films.*
And: One contrast I noticed between the two films:
Whisper of the Heart begins with a view of a city at night.
Grave of the Fireflies ends with a view of a city at night.
(In Grave of the Fireflies, the city is Kobe. In Whisper of the Heart, the city is Tokyo.)
*In Whisper of the Heart, Nishi has to go back to Japan because of the start of the war. In Grave of the Fireflies, Seita continually tries to contact his father, who is in the navy.