Detect using JS Only If element exists on Page and Is Visible

How would I detect if this element is on the current page and visible? Using JavaScript and NOT jQuery.

<p><strong>Before you Start</strong></p>

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I'm fairly sure this function will suffice with detecting if visible.

    function _dtmIsVis(elem) {
        if (elem === null) {
            return false;
        }
        return !!(elem.offsetWidth || elem.offsetHeight || elem.getClientRects().length);
    };

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Thanks!