A bookmarlet source and URL to add a baseline grid to the current page, very useful for CSS development

baseline-grid.js

(function (global) {
  'use strict';

  function createBaselineGrid(height, lines, bgColor, halfLineColor) {
    var 
      halfHeight = height/2,
      grid = document.createElement('div');
    
    grid.id = '___baselineGrid';
    grid.style.position='absolute';
    grid.style.top = 0; 
    grid.style.left = 0; 
    grid.style.right = 0;
    grid.style.opacity = 0.1;
    grid.style.boxSizing = 'border-box';
    grid.style.margin = 0;
    grid.style.padding = 0;
    grid.style.zIndex = -1;
    grid.style.display = 'none';
    
    function createLine(i) {
      var line = document.createElement('div');
      line.style.height = height + 'px';
      line.style.boxSizing = 'border-box';
      line.style.backgroundColor = (i % 2 === 0) ? bgColor : 'transparent';
      
      var half = document.createElement('div');
      half.style.height = halfHeight + 'px';
      half.style.boxSizing = 'border-box';
      half.style.borderBottom = '1px solid ' + halfLineColor;
      
      line.appendChild(half);
      return line;
    } 
    
    for (var i = 0; i <  lines; i++) {
      grid.appendChild(createLine(i));
    }
    
    document.body.appendChild(grid);
    return grid;
  }

  window.__toggleBaselineGrid = function (height, lines, bgColor, halfLineColor) {
    var bg = document.getElementById('___baselineGrid');
    
    if (!bg) {
      height = height || 20;
      lines = lines || 400;
      bgColor = bgColor || 'lightcoral';
      halfLineColor = halfLineColor || 'blue';

      bg = createBaselineGrid(height, lines, bgColor, halfLineColor);
    }
    bg.style.display = bg.style.display === 'none' ? 'block' : 'none';
  };
})(window);

bookmarlet

javascript:(function (d,w) { var s; if (!w.__toggleBaselineGrid){s=d.createElement('script');s.src='https://rawgit.com/dfernandez79/64937704c7d3d7750c45/raw/baseline-grid.js';d.body.appendChild(s);}else{w.__toggleBaselineGrid(20,400);}})(document,window);

To use this bookmarlet add the big bookmarklet URL to your bookmarks.

Please note that in order to use the gist, the bookmarklet is using rawgit.com, which has some usage restrictions... don't abuse of the link.