How did Hyundai-Kia Automotive group end up being so successful after starting from lemon cars?

In the late 1980's and early 1990's Hyundai/Kia cars were considered lemons and were very harshly criticized by many. However today, they offer all kinds of high quality cars with 100,000 miles warranty and have even gone into luxury division like genesis. How did they manage to do it?