I am a web developer. I have been asked by people to work on improving clients dating site design.
My question is that would it be halal for me to do it?
According to the fatwa listed below, you shouldn't work for them because of this ayah in the quran:
but do not help one another in sin and transgression [al-Maa’idah 5:2]
Full fatwa here about a web developer asking about making a website for selling music which is haraam, which inturn means a web developer shouldn't help them achieve their goal. Then I guess the same principles would apply for making a dating website, as dating is haraam.
Is dating allowed?
The Prophet (sws) said: “Whenever a man is alone with a woman the Devil makes a third.” (Sahih Bukhari)
You can read more about dating and it's permissibility in islam here.
Allah Says in Suratul Isra'a Ayah 32:
وَلَا تَقۡرَبُواْ ٱلزِّنَىٰٓۖ إِنَّهُ ۥ كَانَ فَـٰحِشَةً۬ وَسَآءَ سَبِيلاً۬ (٣٢)
Nor come nigh to adultery: for it is a shameful (deed) and an evil, opening the road (to other evils). (32)
Zina is Haram, and in this Ayah it is clear that Allah did not say do not commit Zina, no he says do not even come close to Zina. So not coming close to Zina means blocking everything that will get you there, the eyes which should be lowered, and the gentiles and others. One should not date, rather he should look into marrying. Dating can also help you get to sin Zina, so helping in that sin is and even helping to get another person in their sin is not allowed.
وَلَا تَعَاوَنُواْ عَلَى ٱلۡإِثۡمِ وَٱلۡعُدۡوَٲنِۚ help ye not one another in sin and rancour