After clicking TOP, it does not jump directly to the top of the page, but scrolls up_javascript skills

Change the size of the script in p in the demo to larger or smaller size. . . The time it takes to move to the top is always certain. You can also set a threshold. Before the page height reaches this threshold, the total time it takes to move is. When the page height exceeds this number, the total time it takes to move is based on a certain proportion. Reduce it, try it yourself...

<title>Script House_Back to top effect code</title> <meta http-equiv="Content-Type" content="text/html; charset=gb2312"> <meta name="Author" content="Sheneyan"> <p> </p> <a href="#header"> </a> go to 澶