HEAD:
HEAD / HTTP/1.1 nc.exmaple.com
HEAD is a interesting method, it allow you to get a header of file without get the whole content.
It allows you to check whether there is enought empty space to store the response or the cache is still up to date. This way broswer can avoid redownloading the file since the most recently cache is still valid.
You might not see HEAD in request becasue in GET request also contains HEAD, we don‘t want to do 2 ROUND TRIP, important thing to remember, we need to reduce as much as possilbe to speed up our website.
OPTIONS:
Get number of methods supports in you url. Not every browsers support this.
原文：http://www.cnblogs.com/Answer1215/p/5636139.html