HTML5 CSS not applying

I'm using the following in ASP.net to display an Image, as well css. But when I view the source, it doesn't apply the CSS to the image. I've ran the W3C validator, and keeps telling that the Border attribute is obsolete:

Quote:

---

The border attribute is obsolete. Consider specifying img { border: 0; } in CSS instead.
<img id="ctl00_topmenu" src="_images/menutop1.jpg" alt="topmenu" border="0" />

---

Linking to my CSS

Code:

---

<link href="_css/style.css" rel="stylesheet" type="text/css" />

---

Specifying my image

Code:

---

    <asp:Image ID="topmenu" CssClass="topimg" runat="server" ImageUrl="~/_images/menutop1.jpg" AlternateText="topmenu"/>

---

CSS:

Code:

---

.topimg
{
    vertical-align:top;
    padding:0px;
    border-width:1px;
}

---

And then this is what is displayed in the source of the site :

Code:

---

<img id="ctl00_topmenu" class="topimg" src="_images/menutop1.jpg" alt="topmenu" style="border-width:0px;" />

---

Any idea what am I doing wrong? Just let me know if I need to provide more code..

Thanks

Another thing, even tho I remove the css completely, and also don't specify a border at all, whether in css or on the image self, I still get the border is obsolete error in W3C? And the source still displays this no matter what I do.

Code:

---

<img id="ctl00_topmenu" class="topimg" src="_images/menutop1.jpg" alt="topmenu" style="border-width:0px;" />

---

Have no idea how to get rid of the error, which is my ultimate goal