Is filial piety generally about the parents' satisfaction, or are there fixed guidelines for it? And is the child sinful if he deals harshly with his parents, given that this style is accustomed to them and does not anger them?

Allah and His Messenger commanded dutifulness to parents and forbade disobedience to them. It is stated in the Holy Qur'an: (And your Lord has decreed that you worship none but Him. And to parents, show goodness. If one of them or both of them attain old age in your life, say not to them a word of disrespect, nor repel them, but address them in terms of honor. And lower to them the wing of humility out of mercy, and say, "My Lord, have mercy upon them as they brought me up when I was small.")

Dutifulness to parents has established guidelines, including: it must be within what is good, meaning there is no obedience to a creature in disobedience to the Creator; and it must be with good conduct, by adopting an approach of respect, veneration, and deference, lowering one's voice when speaking, choosing the kindest words, and seeking the most tender meanings.

One must also avoid bad manners such as raising one's voice, using vulgar language, or a harsh manner. In dutifulness to them, one seeks obedience to Allah, not merely obedience to them.

There are matters for which no specific ruling has been mentioned in Islamic law, but which people in a particular place or time agree constitute bad manners and conduct. One must refrain from these with parents, even if they do not show anger.

As for matters that people are accustomed to and do not consider bad manners, there is no harm in them as long as they do not constitute harshness or roughness. And for what wavers between these two, and where people's customs are confused, the requirement of good manners with parents is to refrain from it.