html2canvas is not defined

Question

I'm developing my project using angularjs and php and need to save a certificate which will be generated at the end, Certificate with it's contents is created dynamically but cannot do right click->save as since it is not an image, So I used canvas for this, but it doesn't seem to help.

I included this in head tag of my HTML file

<script src="https://cdnjs.cloudflare.com/ajax/libs/html2canvas/0.5.0-alpha1/html2canvas.js">
</script>

But still it says "html2canvas is not defined"

MY HTML CODE:

<div class="userCertificate" id="html-content-holder">
    <div class="secondborder">
        <img src="images/ISRO_Logo.png" height="80px" alt="ISRO_LOGO" style="text-align: left">
        <div class="content">
            <h2 style="color:#f47216">NATIONAL REMOTE SENSING CENTRE</h2>
            <br>
            <h1>Certificate Of Appreciation</h1>
            <br>
            <p>This certificate is awarded to </p>
            <h1><span class="Name"> {{studentname}} </span></h1>
            <p>from <span class="collegeName">{{collegeName}}</span> for succesfully qualifying online quiz exam with score of <span class="collegeName">{{marks}}</span> %</p>
            <p> conducted on 6/06/2019</p>
        </div>
        <img src="images/stamp.PNG" height="80px" alt="ISRO_LOGO" style="float: right;margin-top: 80px;margin-right: 30px">
    </div>
</div>

<input type='button' id='but_screenshot' value='Take screenshot' onclick='screenshot();'>

My Javascript Code:

function screenshot() {
            html2canvas(document.getElementById('userCertificate')).then(function(canvas) {
                document.body.appendChild(canvas);
            });
        }