Css – asp:button disappears in IE7, but stays in IE8. Why

I have the following html which is display correctly in IE8. Below is the html and the css for grey_btn_75. Any idea why this would be happening?

<div style="float: left; width: 70px; margin-right: 25px; padding-top: 60px;
            margin-left: 25px">

<asp:Button ID="btnAddAll" runat="server" Text="Add All" 
            OnClick="btnAddAll_Click"
            CssClass="grey_btn_75" />

<div class="spacer"></div>

<asp:Button ID="btnAdd" runat="server" Text="Add"
            OnClick="btnAdd_Click" 
            CssClass="grey_btn_75" />

<div class="spacer"></div>

<asp:Button ID="btnRemove" runat="server" Text="Remove" 
            OnClick="btnRemove_Click"
            CssClass="grey_btn_75" />

<div class="spacer"></div>

<asp:Button ID="btnRemoveAll" runat="server" Text="Remove All" 
            CssClass="grey_btn_75"
            OnClick="btnRemoveAll_Click" /><br />

</div>

CSS:

.grey_btn_75
{
background: url(../images/grey-75px.png);
background-repeat: no-repeat;
border-style: none;
font-family: Arial,Helvetica,Sans-Serif;
font-size: 12px;
font-weight: bold;
width: 75px;
height: 23px;
color: #000000;
cursor: pointer;
}

Things I have tried so far:

I removed the CssClass and the buttons still did not show up.
I modifed the CssClass and the buttons still did not show up.
I put other controls such as an asp:Label and and asp:ImageButton and they showed up fine.
I tried putting a new button and it did not show up.