Is it permissible for a woman to prostrate if her prayer garment covers three-quarters of her forehead, showing only a very small part of it, due to the covering of her hairline?

It is sufficient for prostration to occur with the entire forehead, or most of it, covered by the jilbab (outer garment). Therefore, it is not obligatory for the worshipper to directly touch the ground with any of these limbs. If one prostrates on a turban's fold, or one's sleeve, or the hem of one's garment, the prayer is valid.