CSS is dynamically centered relative to the browser

Use the following CSS to keep the div in the middle of the screen forever

<style type= "Text/css" >                . log {            position:absolute;   /* Absolute positioning */            top:50%;             /* 50%*/left:50% from top            ;              margin:-100px 0 0-150px;      /* Set the margin-top of this div to a negative value of one half of its height, and margin-left is a negative half of its width. (feeling in tongue twisters) */            width:300px;                              /* Width is 400, then Margin-top is -200px*/            height:200px;                            /* High for 200 then margin-left for -100px;*/            z-index:99;                            /* Floating at the top */            }    </style>

CSS is dynamically centered relative to the browser