Chrome Dev Tools Snippet

grid-overlay.js

function grid(container, columns) {
  'use strict';
  var els = document.querySelectorAll(container);

  [].forEach.call(els, function(el){
    var frag = document.createDocumentFragment(),
        grid = document.createElement('div'),
        cols = columns;

    grid.style.position   = 'absolute';
    grid.style.left       = 0;
    grid.style.top        = 0;
    grid.style.width      = '100%';
    grid.style.height     = '100%';
    grid.style.borderLeft = '1px solid #3bffff';
    grid.style.overflow   = 'hidden';
    grid.style.pointerEvents = 'none';
    frag.appendChild(grid);

    for (var i = 0; i < cols; i++) {
        var div = document.createElement('div');

        // div.style.borderLeft  = '1px solid blue';
        div.style.borderRight = '1px solid #3bffff';
        div.style.width       = 100 / columns + '%';
        div.style.height      = '100%';
        div.style.float       = 'left';
        div.style.boxSizing   = 'border-box';
        grid.appendChild(div);
    }
    el.appendChild(frag);
  });
}

// grid('.container', 20);