<!DOCTYPE html>

<math>
    <mfrac id="mfrac"><mn>1</mn><mn>2</mn></mfrac>
    
    <msqrt id="msqrt"><mi>a</mi><mi>b</mi><mi>c</mi></msqrt>
    
    <mn id="mn4">4</mn>
    <mn id="mn5">5</mn>
    <mn id="mn6">6</mn>
    <mn id="mn7">7</mn>
</math>

<script>
    document.body.offsetTop;
    
    var mfrac = document.getElementById("mfrac");
    mfrac.replaceChild(document.getElementById("mn4"), mfrac.lastChild);
    mfrac.replaceChild(document.getElementById("mn5"), mfrac.firstChild);
    
    var msqrt = document.getElementById("msqrt");
    msqrt.replaceChild(document.getElementById("mn6"), msqrt.firstChild);
    msqrt.replaceChild(document.getElementById("mn7"), msqrt.lastChild);
</script>