maximizeFontSize.js

function maximizeFontSize(elem, maxWidth, maxHeight, opt_dontChangeFontSize) {
  var canBeBigger,
      display = elem.style.display,
      fontSize = elem.style.fontSize,
      font = elem.style.font,
      computedFont = window.getComputedStyle(elem, null).getPropertyValue('font'),
      doc = elem.ownerDocument,
      parentNode = elem.parentNode,
      tempParent = doc.createElement('div'),
      nextSibling = elem.nextSibling,
      newFontSize = 0.5,
      diff = 0.5;
  
  elem.style.display = 'inline-block';
  elem.style.font = computedFont;
  
  tempParent.style.display = 'block';
  tempParent.style.width = maxWidth;
  tempParent.style.height = maxHeight;
  tempParent.style.overflow = 'hidden';
  
  doc.body.appendChild(tempParent);
  tempParent.appendChild(elem);
  
  do {
    newFontSize += diff;
    elem.style.fontSize = newFontSize + 'px';
    if (canBeBigger = (elem.clientWidth <= maxWidth && elem.clientHeight <= maxHeight)) {
      diff *= 2;
    }
    else {
      newFontSize -= diff;
      diff /= 2;
    }
  } while((!canBeBigger && newFontSize > 0.5) || diff >= 1);
  
  if (opt_dontChangeFontSize || !canBeBigger) {
    elem.style.fontSize = fontSize;
  }
  
  doc.body.removeChild(tempParent);
  parentNode.insertBefore(elem, nextSibling);
  elem.style.display = display;

  if (font) {
    elem.style.font = (opt_dontChangeFontSize || !canBeBigger) ? font : font.replace(/\b(\d+(\.\d+)?|\.\d+)[A-Za-z]+\b/, newFontSize + 'px');
  }
  else {
    delete elem.style.font;
  }
  
  return canBeBigger ? newFontSize : undefined;
}