According to Trent Lorcher in "Lesson Plans: Modernism in Literature," modernism is:
...marked by a strong and intentional break with tradition. This break includes a strong reaction against established religious, political, and social views.
In "A Rose For Emily," Faulkner uses a modernist style in laying aside the perceptions of the proper Southern woman, a member of polite society. He presents—"speaking" as a member of the community—the impressions the townspeople have of Emily who is the daughter of a well-to-do leader in the community in which Emily still lives, many years after the death of her family and peers.
Where authors previously chose to write in the Realist mode, with modernism, writing took on topics that questioned the status quo and presented themes that would have been considered unsuitable or "inappropriate" by earlier writers and audiences.
Faulkner writes a story of a woman who shatters the perceptions the townspeople have of the culturally and economically elite: members of society who have been put up on a pedestal, who are "better" than the common folk.
Emily does not follow the dictates of society: she is seen riding around in an open carriage with Homer Baron, a member of the working class, and a highly visible bachelor who would be perceived as being beneath her.
Emily also is unapproachable about the smell emanating from her home. The town's "elders" are unable to get her to speak to them about taxes, later, about the odor. She is a law unto herself. This would have been expected of a woman of her status: in the absence of a man in her home, she lives alone and does not rely on a man for her survival in any way. The men of the society would have expected her to marry, but the women would have understood that her father made it impossible, as no man was ever good enough. Because she was a member of the upperclass, everyone would have excused her unusual behavior.
Faulkner's "unmasking" of Emily's "real" self is the true mark of the modernist style of writing. All the town's (and readers') preconceived notions of Emily are disproven—literally blown apart—and Faulkner achieves this by not only methodically giving brief glimpses of Emily's existence, but telling her story while jumping around on the timeline of her life.
The truly taboo subject that would have stunned readers in 1930 when it was first published (and still stuns readers today) is not that Emily has murdered her lover, Homer Baron, but that she was sleeping in the same bed with his corpse LONG after his death: this is evident with one small detail...the single strand of steel grey hair resting on the pillow next to the dead body's head.