Im planning a trip to taiwan at end of dec. Hows the weather and any clothing recommendations?

Except the July, August, January and February, other monthes are all good time to travel to that place.
December is cold time for the island, but the average temperature is around 17C in Taipei for that month. It is suggested to take your thick sweaters and thick coat. If you have thin down jacket, you may take one. It will be humid during that time.

Having lived in North Eastern US for many years I actually prefer Taiwan during the "winter' months of Dec, Jan, Feb. I see Taiwanese people bundled up in heavy winter jacket when temp falls to mid teens but I am usually fine with a long sleeve shirt. At most a light jacket, unless you are going to take long rides on a motor scooter.

Do take or purchase a compact umbrella for your trip. (You can find good quality compact umbrellas for ~$6 USD) I was in Taiwan/Taipei for 2 weeks the end of 2012 and it rained nearly every day. Went back at the end of January to March and it was clear every single day. By early March there were talks of water rationing in some counties.