Exemple de code de la propriété CSS border-top-width.

<style type="text/css">
.default{ border-color : #666; border-style : solid; margin : 10px 0; padding :2px; }
.exemple-border-top-width-3px{ border-top-width : 3px }
.exemple-border-top-width-1em{ border-top-width : 1em }
.exemple-border-top-width-1rem{ border-top-width : 1rem }
.exemple-border-top-width-thin{ border-top-width : thin }
.exemple-border-top-width-medium{ border-top-width : medium }
.exemple-border-top-width-thick{ border-top-width : thick }  
</style>

<div class="default exemple-border-top-width-3px">border-top-width : 3px </div> 
<div class="default exemple-border-top-width-1em">border-top-width : 1em </div>
<div class="default exemple-border-top-width-1rem">border-top-width : 1rem </div> 
<div class="default exemple-border-top-width-thin">border-top-width : thin </div>
<div class="default exemple-border-top-width-medium">border-top-width : medium </div> 
<div class="default exemple-border-top-width-thick">border-top-width : thick </div>