//get the header ID element

document.querySelector("#header");

//get the first element with a dropcap class

document.querySelector(".dropcap");

//get all the paragraphs with a "dropcap" class produces a nodelist

document.querySelectorAll(".dropcap");

//get all elements with a class of "dropcap" or "huge"

document .querySelectorAll(".dropcap,.huge");

//get all paragraphs that hava a class

document.querySelectorAll("p[class]");

浏览器支持度

IE	8.0及以上
Firefox	3.5及以上
chrome	1及更高
opera	10及更高
Safari	3.2及更高