elem.style.xxx 只能获取内联的css属性，对<head>内的或link 外部样式的无能为力。
<style> h1{ font-size:1.6em; color:red; } </style> </head> <body> <h1 id="title">hello world</h1> <p>this is a test txt</p> <script> var x=document.getElementById("title"); document.write(x.style.color); </script>
其实
x.style.color="";
所以看不到样式信息.
A function for finding the computed style value of an element is shown in Listing 7-1
// Get a style property (name) of a specific element (elem) function getStyle( elem, name ) { // If the property exists in style[], then it's been set // recently (and is current) if (elem.style[name]) return elem.style[name]; // Otherwise, try to use IE's method
else if (elem.currentStyle) return elem.currentStyle[name]; // Or the W3C's method, if it exists
else if (document.defaultView && document.defaultView.getComputedStyle) { // It uses the traditional 'text-align' style of rule writing, // instead of textAlign name = name.replace(/([A-Z])/g,"-$1"); name = name.toLowerCase(); // Get the style object and get the value of the property (if it exists) var s = document.defaultView.getComputedStyle(elem,""); return s && s.getPropertyValue(name); // Otherwise, we're using some other browser } else return null; }
w3c办法，它使用的是通用的text-align而非textAlign；所以先要把属性名替换。
注意；几乎任何属性都会返回表示元素样式的字符串而非数值（比如是100px而非100）；
<html> <head> <style>p { height: 100px; }</style> <script> window.onload = function(){ // Locate the paragraph to check the height of var p = document.getElementsByTagName(“p”)[0]; // Check the height the traditional way alert( p.style.height + “ should be null” ); // Check the computed value of the height alert( getStyle( p, “height” ) + “ should be 100px” ); }; </script> </head> <body> <p>I should be 100 pixels tall.</p> </body> </html>
上面展示了如何获取一个dom元素的css属性的最终 真实值。 需要注意的是：这个函数忽略的其他的计量单位（如百分比）。所以这个方法并不完美，但已经是一个很好的开端。
chrome 下 height:x %;
不过是x是什么，都是18。
Ie的currentStyle 对象 代表了在全局样式表、内嵌样式和 HTML 标签属性中指定的对象格式和样式。http://msdn.microsoft.com/en-us/library/ie/ms535231(v=vs.85).aspx
W3c ：
document.defaultView
browsers returns the window object associated with the document ornullif none available
var win = document.defaultView;getComputedStyle()gives the final used values of all the CSS properties of an element.
var style = window.getComputedStyle(element[, pseudoElt]);
The returnedstyleis aCSSStyleDeclarationobject.
pseudoElt Optional
A string specifying the pseudo-element to match. Must benull(or not specified) for regular elements.
通常的元素必须为null。
CSSStyleDeclaration 方法
getPropertyValue(propertyName)
removeProperty(propertyName)
参考：https://developer.mozilla.org/en-US/docs/DOM/window.getComputedStyle