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HTML

<h1 id="heading1" style="text-align:center; height:auto; width:auto; font-family:'Arial Black', Gadget, sans-serif">Determinant of a nxn matrix</h1> 
<p id="paragraph1" style="font-family:'Arial Black', Gadget, sans-serif"> This program allows you to compute the determinant of a nxn matrix</p>

<p>
    Input the order of the matrix
    <br />
    
    <input type="text" maxlength="3" name="value" id='value' />
    <input type="button" value="submit" onclick="verifyorder()" />
</p>
<p id="error"></p>
<p id="detspace"></p>

JavaScript

function verifyorder() {
    var order = document.getElementById('value').value;
    if (+order > 0) {
        alert(+order);
        return true;
    }
    else {
        alert("Sorry, you need to enter a positive interger value, try again");
        document.getElementById('error').innerHTML = "Sorry, you need to enter a positive interger value, try again";
    }
}