html2canvas/src/font.js

function Font(family, size) {
    var container = document.createElement('div'),
        img = document.createElement('img'),
        span = document.createElement('span'),
        sampleText = 'Hidden Text',
        baseline,
        middle;

    container.style.visibility = "hidden";
    container.style.fontFamily = family;
    container.style.fontSize = size;
    container.style.margin = 0;
    container.style.padding = 0;

    document.body.appendChild(container);

    img.src = smallImage();
    img.width = 1;
    img.height = 1;

    img.style.margin = 0;
    img.style.padding = 0;
    img.style.verticalAlign = "baseline";

    span.style.fontFamily = family;
    span.style.fontSize = size;
    span.style.margin = 0;
    span.style.padding = 0;

    span.appendChild(document.createTextNode(sampleText));
    container.appendChild(span);
    container.appendChild(img);
    baseline = (img.offsetTop - span.offsetTop) + 1;

    container.removeChild(span);
    container.appendChild(document.createTextNode(sampleText));

    container.style.lineHeight = "normal";
    img.style.verticalAlign = "super";

    middle = (img.offsetTop-container.offsetTop) + 1;

    document.body.removeChild(container);

    this.baseline = baseline;
    this.lineWidth = 1;
    this.middle = middle;
}