Well, I went through every solution I could find.
The Ajax Control Toolkit works, but it creates a weird html output with all kinds of spans and other styling that is hard to work with.
Using css styling with the ::before
tags to hide the original control's box would work, but if you placed runat=server
into the element to make it accessible to the code-behind, the checkbox would not change values unless you actually clicked in the original control.
In some of the methods, the entire line for the label would end up under the checkbox if the text was too long for the viewing screen, or would end up underneath the checkbox.
In the end, (on the adice of @dimarzionist's answer here in this page) I used an asp.net ImageButton and used the codebehind to change the image. With this solution I get nice control over the styles and can determine whether the box is checked from the codebehind.

<asp:ImageButton ID="mycheckbox" CssClass="checkbox" runat="server" OnClick="checkbox_Click" ImageUrl="unchecked.png" />
<span class="checkboxlabel">I have read and promise to fulfill the <a href="/index.aspx">rules and obligations</a></span>
And in the code-behind
protected void checkbox_Click(object sender, ImageClickEventArgs e) {
if (mycheckbox.ImageUrl == "unchecked.png") {
mycheckbox.ImageUrl = "checked.png";
//Do something if user checks the box
} else {
mycheckbox.ImageUrl = "unchecked.png";
//Do something if the user unchecks the box
}
}
What's more, is with this method, The <span>
you use for the checkbox's text will wrap perfectly with the checkbox.

.checkboxlabel{
vertical-align:middle;
font-weight: bold;
}
.checkbox{
height: 24px; /*height of the checkbox image*/
vertical-align: middle;
}