Edit in JSFiddle

YUI().use('node', 'rounded-corners', function(Y){ Y.on('domready', function(){
    Y.one('#text1').plug(Y.Plugin.NekoCM.RoundedCorners);
    Y.one('#text2').plug(Y.Plugin.NekoCM.RoundedCorners, {
        br: false,
        bl: false
    });
    Y.one('#text3').plug(Y.Plugin.NekoCM.RoundedCorners, {
        tr: false,
        tl: false
    });
    Y.one('#text4').plug(Y.Plugin.NekoCM.RoundedCorners, {
        tl: false,
        br: false,
        bl: false
    });
    Y.one('#text5').plug(Y.Plugin.NekoCM.RoundedCorners, { 
        radius: 5
    });
    Y.one('#text6').plug(Y.Plugin.NekoCM.RoundedCorners, {
        radius: 10
    });
})});

<div class="box" id="text1">Box</div>
<div class="box" id="text2">Only top</div>
<div class="box" id="text3">Only bottom</div>
<div class="box" id="text4">Only tr</div>
<div class="box" id="text5">Radius 5</div>
<div class="box" id="text6">Radius 10</div>

div.box {
    background-color: #333;
    color: white;
    padding: 5px;
    display: inline-block;
}