A CodePen by Daniel Schmidt.

index.html

<canvas id="404" width="600" height="600"></canvas>

script.js

var canvas  = document.getElementById('404');
var context = canvas.getContext('2d');

var time = Date.now();
var initFontSize = 20;
var maxPosX = 420.0;
var minPosX = 10;

function clear() {
  context.clearRect(0, 0, canvas.width, canvas.height);
}

function Message(context, posX, posY){
  this.context = context; 
  this.posX    = posX;
  this.posY    = posY;
  
  this.messageSize = "30px";
}

Message.prototype.shout = function(text){
  this.context.webkitImageSmoothingEnabled = true;
  this.context.fillStyle = "white";
  this.context.font= this.messageSize + " Freckle Face";
  this.context.fillText(text,this.posX,this.posY); 
  this.context.strokeStyle = 'black';
  this.context.strokeText(text,this.posX,this.posY); 
}
var message = new Message(context,180,65);
var reachedRight = false;
var reachedLeft = true;
function scaleError(){
  clear();
  
  var newX = ((Date.now() - time) / 1000);
 
  if(message.posX <= maxPosX && !reachedRight){
    message.posX += newX;
    var newFontSize = (initFontSize + newX)*2.5;
    message.messageSize = newFontSize+"px";
  }else{
    if(message.posX >= maxPosX){
      reachedRight = true;
      reachedLeft = false;
    }
    //message.posX -= newX;
  }
  
  if(message.posX <= minPosX){
    reachedRight = false;
  }
  
  if(reachedRight){
    message.messageSize = "60px";
  }
  
  
  message.shout("404");
}

setInterval(scaleError,50);

style.css

@import url(http://fonts.googleapis.com/css?family=Freckle+Face);

body{
  font-family: 'Freckle Face', cursive;
}

canvas{
  border:1px solid black;
  background-color: #eeeeee;
}