Why do birthmarks appear and fade? Are they different from port wine stains?

[frankness]

I have read that birthmarks are caused by abnormalities in the development of the blood vessels, but my question refers specifically to Haemangioma. They appear a few days after birth, the skin becomes quite raised and red, only to fade after a few years.
I was wondering why they take a few days to appear and then fade? Why are they different from port wine stains which are flat and present from birth?
Is there any reason why these marks are often on the face rather than other parts of the body?

 

[Moonbear]

I don't know anything about port wine stains other than they exist.

According to this site http://www.chw.edu.au/parents/factsheets/haemangj.htm
the cause of haemangiomas is unknown, which makes it hard to guess at a good reason for their location.

 

[baywax]

http://www.google.ca/search?hl=en&q=portwine+stains&btnG=Google+Search&meta=

http://www.google.ca/search?hl=en&q=Haemangioma&btnG=Search&meta=

Even if you can't spell it, you can google it.

 

[frankness]

Thanks.
I had googled it but could only find the "what" and I was really wondering "why?". Guess it's not been cracked yet, but it's not exactly the most important medical mystery.