整个网页置灰的办法
直接看例子：
<html xmlns="http://www.w3.org/1999/xhtml"
style="
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-ms-filter: grayscale(100%);
-o-filter: grayscale(100%);
filter: grayscale(100%);
filter: progid:DXImageTransform.Microsoft.BasicImage(grayscale=1);">
......
</html>