losing changes in the editor

When i add an style using HTML mode in Visual Editor in the header like this:

<style>

body {

background: url(images/escudo.jpg) no-repeat center top fixed;

-webkit-background-size: 50px;

-moz-background-size: 50px;

-o-background-size: 50px;

background-size: 50px;

}

</style>

If I return to Design mode, lose <style> and </style> tags.

Alberto