访问DOM元素
DOM节点概述
// Returns a reference to the element by its ID.
document . getElementById ( "someid" ) ;
// Returns an array-like object of all child elements which have all of the given class names.
document . getElementsByClassName ( "someclass" ) ;
// Returns an HTMLCollection of elements with the given tag name.
document . getElementsByTagName ( "LI" ) ;
// Returns the first element within t
