html2canvas/src/Font.js

_html2canvas.Util.Font = (function () {

  var fontData = {};

  return function(font, fontSize, doc) {
    if (fontData[font + "-" + fontSize] !== undefined) {
      return fontData[font + "-" + fontSize];
    }

    var container = doc.createElement('div'),
    img = doc.createElement('img'),
    span = doc.createElement('span'),
    sampleText = 'Hidden Text',
    baseline,
    middle,
    metricsObj;

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

    doc.body.appendChild(container);

    // http://probablyprogramming.com/2009/03/15/the-tiniest-gif-ever (handtinywhite.gif)
    img.src = "data:image/gif;base64,R0lGODlhAQABAIABAP///wAAACwAAAAAAQABAAACAkQBADs=";
    img.width = 1;
    img.height = 1;

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

    span.style.fontFamily = font;
    span.style.fontSize = fontSize;
    span.style.margin = 0;
    span.style.padding = 0;

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

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

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

    middle = (img.offsetTop-container.offsetTop) + 1;
    metricsObj = {
      baseline: baseline,
      lineWidth: 1,
      middle: middle
    };

    fontData[font + "-" + fontSize] = metricsObj;

    doc.body.removeChild(container);

    return metricsObj;
  };
})();