JSFiddle - React, Tailwind, and code Playground

CSS

body {
    background-color: white;
    margin: 0;
    text-align: center;
    overflow: hidden;
}
.grid {
    display: inline-block;
    background-color: blue;
}
.cellDiv:nth-child(odd) {
    background-color: red;
    float:left;
}
.cellDiv:nth-child(even) {
    background-color: green;
    float:left;
}

JavaScript

console.log('width: ' + window.innerWidth, 'height: ' + window.innerHeight)

  var windowWidth = window.innerWidth;
  var windowHeight = window.innerHeight;
  var size = 10;

function closestMultiple(width, size, n, limit) {
    if(n > limit) {
        return {m: width/size, s:size};
    }
    if((width % (size+n)) == 0) {
        return {m: width / (size+n), s: size+n};
    } else if((width % (size-n)) == 0) {
        return {m: width / (size-n), s: size-n};
    }

    return closestMultiple(width, size, n+1, limit);
}
  var wcm = closestMultiple(windowWidth, size, 0, 5);
  var hcm = closestMultiple(windowHeight, size, 0, 5);

  // Calculate the number of cells we can fit in the width 
  //and height (there will be extra space)
  var w = Math.floor(wcm.m);
  var h = Math.floor(hcm.m);
console.log(wcm);
console.log(hcm);

  // Calculate the extra space
  var widthDiff = windowWidth % wcm.s;
  var heightDiff = windowHeight % hcm.s;

  // Add the needed amount of height and width to each cell to fill the window
  var widthSize = Math.floor(wcm.s + widthDiff / w);
  var heightSize = Math.floor(hcm.s + heightDiff / h);

  //NOW, convert pixels to percentages.
  var widthPercent = widthSize / windowWidth * 100;
  var heightPercent = heightSize / windowHeight * 100;

  // Begin to alter the DOM
  var parentDiv = document.createElement('div');
  parentDiv.className = 'grid';
  parentDiv.style.height = windowHeight + 'px';
  parentDiv.style.width = windowWidth + 'px';

  for(var x = 0; x < w*h; x++) {
    var cellDiv = document.createElement('div');
    cellDiv.className = 'cellDiv';
    cellDiv.style.height = heightPercent + '%'; 
    cellDiv.style.width = widthPercent + '%'; 
    parentDiv.appendChild(cellDiv)
  }
  document.body.appendChild(parentDiv);