Drawing a grid using CSS

Here's a simple solution using jQuery. This script will try to fill in as many grid element as possible without overflowing. The function accepts a single parameter, which defines the size of the grid.

function createGrid(size) {
    var ratioW = Math.floor($(window).width()/size),
        ratioH = Math.floor($(window).height()/size);

    var parent = $('<div />', {
        class: 'grid',
        width: ratioW  * size,
        height: ratioH  * size
    }).addClass('grid').appendTo('body');

    for (var i = 0; i < ratioH; i++) {
        for(var p = 0; p < ratioW; p++){
            $('<div />', {
                width: size - 1,
                height: size - 1
            }).appendTo(parent);
        }
    }
}

It also requires a simple CSS style:

.grid {
    border: 1px solid #ccc;
    border-width: 1px 0 0 1px;
}

.grid div {
    border: 1px solid #ccc;
    border-width: 0 1px 1px 0;
    float: left;
}

See a simple demo here: http://jsfiddle.net/yijiang/nsYyc/1/

Here's one using native DOM functions. I should also change the initial ratio calculation to use DOM functions but I cannot for the life of me get window.innerWidth to return accurate numbers fixed that:

function createGrid(size) {
    var ratioW = Math.floor((window.innerWidth || document.documentElement.offsetWidth) / size),
        ratioH = Math.floor((window.innerHeight || document.documentElement.offsetHeight) / size);

    var parent = document.createElement('div');
    parent.className = 'grid';
    parent.style.width = (ratioW * size) + 'px';
    parent.style.height = (ratioH * size) + 'px';

    for (var i = 0; i < ratioH; i++) {
        for (var p = 0; p < ratioW; p++) {
            var cell = document.createElement('div');
            cell.style.height = (size - 1) + 'px';
            cell.style.width = (size - 1) + 'px';
            parent.appendChild(cell);
        }
    }

    document.body.appendChild(parent);
}

createGrid(10);

It's basically a direct translation of the jQuery code. If you need even more performance you can switch to generating the boxes using strings pushed to an array:

arr.push('<div style="width:', (size - 1), 'px;height:', (size - 1), 'px;"></div>');

then at the end

parent.innerHTML = arr.join('');

Here is a simple CSS-only solution, using linear gradients:

html, 
body,
.grid {
    height: 100%;
    width: 100%;
    margin: 0;
}
.grid {
    background-image:
      repeating-linear-gradient(#ccc 0 1px, transparent 1px 100%),
      repeating-linear-gradient(90deg, #ccc 0 1px, transparent 1px 100%);
    background-size: 71px 71px;
}

<div class="grid"></div>