Hi, I need to make this show an error if the user tries to leave the page without checking this tickbox. It has no other use other than visual, is there a way to implement such a thing?

<span>
        <input value="1"  type="checkbox" name="salgsvilkar" ID="checkbox2"  style="float:left;"
        onclick="document.getElementById('scrollwrap').style.cssText='
border-color:#85c222; 
background-color:#E5F7C7;';" />
<label for="checkbox2" class="akslabel">
Salgs og leveringsvilkår er lest og akseptert
</label>
     </span>

CSS is just for styling.

Thanks.