css font-size 和 line-height 与基线不匹配

A function to measure could be based on <canvas> and look like this :

function getMetrics(fontName, fontSize) {
  // NOTE: if there is no getComputedStyle, this library won't work.
  if(!document.defaultView.getComputedStyle) {
    throw("ERROR: 'document.defaultView.getComputedStyle' not found. This library only works in browsers that can report computed CSS values.");
  }
  if (!document.querySelector('canvas')) {
    var _canvas = document.createElement('canvas');
    _canvas.width = 220; _canvas.height = 220;
    document.body.appendChild(_canvas);
  }
  // Store the old text metrics function on the Canvas2D prototype
  CanvasRenderingContext2D.prototype.measureTextWidth = CanvasRenderingContext2D.prototype.measureText;
  /**
   *  Shortcut function for getting computed CSS values
   */
  var getCSSValue = function(element, property) {
    return document.defaultView.getComputedStyle(element,null).getPropertyValue(property);
  };
  /**
   * The new text metrics function
   */
  CanvasRenderingContext2D.prototype.measureText = function(textstring) {
    var metrics = this.measureTextWidth(textstring),
        fontFamily = getCSSValue(this.canvas,"font-family"),
        fontSize = getCSSValue(this.canvas,"font-size").replace("px",""),
        isSpace = !(/S/.test(textstring));
        metrics.fontsize = fontSize;

    // For text lead values, we meaure a multiline text container.
    var leadDiv = document.createElement("div");
    leadDiv.style.position = "absolute";
    leadDiv.style.margin = 0;
    leadDiv.style.padding = 0;
    leadDiv.style.opacity = 0;
    leadDiv.style.font = fontSize + "px " + fontFamily;
    leadDiv.innerHTML = textstring + "<br/>" + textstring;
    document.body.appendChild(leadDiv);
    // Make some initial guess at the text leading (using the standard TeX ratio)
    metrics.leading = 1.2 * fontSize;
    // Try to get the real value from the browser
    var leadDivHeight = getCSSValue(leadDiv,"height");
    leadDivHeight = leadDivHeight.replace("px","");
    if (leadDivHeight >= fontSize * 2) { metrics.leading = (leadDivHeight/2) | 0; }
    document.body.removeChild(leadDiv);
    // if we're not dealing with white space, we can compute metrics
    if (!isSpace) {
        // Have characters, so measure the text
        var canvas = document.createElement("canvas");
        var padding = 100;
        canvas.width = metrics.width + padding;
        canvas.height = 3*fontSize;
        canvas.style.opacity = 1;
        canvas.style.fontFamily = fontFamily;
        canvas.style.fontSize = fontSize;
        var ctx = canvas.getContext("2d");
        ctx.font = fontSize + "px " + fontFamily;

        var w = canvas.width,
            h = canvas.height,
            baseline = h/2;

        // Set all canvas pixeldata values to 255, with all the content
        // data being 0. This lets us scan for data[i] != 255.
        ctx.fillStyle = "white";
        ctx.fillRect(-1, -1, w+2, h+2);
        ctx.fillStyle = "black";
        ctx.fillText(textstring, padding/2, baseline);
        var pixelData = ctx.getImageData(0, 0, w, h).data;

        // canvas pixel data is w*4 by h*4, because R, G, B and A are separate,
        // consecutive values in the array, rather than stored as 32 bit ints.
        var i = 0,
            w4 = w * 4,
            len = pixelData.length;

        // Finding the ascent uses a normal, forward scanline
        while (++i < len && pixelData[i] === 255) {}
        var ascent = (i/w4)|0;

        // Finding the descent uses a reverse scanline
        i = len - 1;
        while (--i > 0 && pixelData[i] === 255) {}
        var descent = (i/w4)|0;

        // find the min-x coordinate
        for(i = 0; i<len && pixelData[i] === 255; ) {
          i += w4;
          if(i>=len) { i = (i-len) + 4; }}
        var minx = ((i%w4)/4) | 0;

        // find the max-x coordinate
        var step = 1;
        for(i = len-3; i>=0 && pixelData[i] === 255; ) {
          i -= w4;
          if(i<0) { i = (len - 3) - (step++)*4; }}
        var maxx = ((i%w4)/4) + 1 | 0;

        // set font metrics
        metrics.ascent = (baseline - ascent);
        metrics.descent = (descent - baseline);
        metrics.bounds = { minx: minx - (padding/2),
                           maxx: maxx - (padding/2),
                           miny: 0,
                           maxy: descent-ascent };
        metrics.height = 1+(descent - ascent);
    } else {
        // Only whitespace, so we can't measure the text
        metrics.ascent = 0;
        metrics.descent = 0;
        metrics.bounds = { minx: 0,
                           maxx: metrics.width, // Best guess
                           miny: 0,
                           maxy: 0 };
        metrics.height = 0;
    }
    return metrics;
  };

注意，您还需要一个好的reset.css"来重置浏览器边距和填充.
您点击显示 CSS"，还可以使用生成的 CSS 混合多种字体:
如果它们具有不同的基本尺寸，则标准化第二个:

var factor = CSS1baseSize / CSS2baseSize;

var size = size * factor;