Category Archives: CSS

Playing multiple videos simultaneously using HTML5

I was interested to see how playing multiple videos simultaneously affected the performance of a web page. This probably seems a little like a time machine back to 1995 – I’ll soon be posting about flashing red text and Dreamweaver!

However, playing short videos with only a few frames can be a way to draw attention to a particular part of the page; for example, if you’re browsing a clothing catalogue and one of the models moves when you hover over.

It’s worth pointing out that the entire page will be local, and so I have no network considerations whatsoever. This isn’t, however, about having three Netflix streams running at the same time – it’s short, and small videos.

The video that I’m using came from here. I’m creating an effect whereby you hover over an image of a space ship and it explodes.


It’s worth bearing in mind that most video formats are not supported by most browsers. MP4 is the exception, so it looks like this is the way to go for video encoding. I used this tool to convert the file.


Here’s the HTML for a single icon:

  <script src="HoverAnimate.js"></script>
<body onload="loaded()">
  <div id="ship1" style="background-image: url('assets/titan.png'); background-repeat: no-repeat; background-size: 50px 50px; width: 50px; height: 50px">
    <video  width="50" height="50" id="ship1-video" 
            style="visibility: hidden">
      <source src="assets/explosion.mp4" type="video/mp4" />

Clearly this could be neater if the CSS was separated, but essentially what we have is a div element with a scaled background image, which contains a video (currently hidden). The next thing is the Javascript that plays the video:

function loaded() {
    var imgs = document.getElementsByTagName("div");
    [], function (item) {
        if ( !== "") {
            item.addEventListener('mouseover', hoverImg, false);

function hoverImg(e) {      
    var vid = document.getElementById( + "-video");
    vid.onended = function() {        = 'hidden';
    = 'visible';;

This iterates through all the div elements and, for those that has a background image, hooks up a hover event. I’ve also assumed that the div element will be named using the format “{imagename}-video”.


I tried this with seven videos simultaneously, and didn’t see any jerking of the animations. Whether this would stand up under networked conditions, it’s hard to say, but with the video locally available, performance is fine.


Create CSS effect to “Shine” a button border

Imagine that you have an HTML button or element on a page and you would like an effect where the border shines all around the perimeter. this provides an excellent example of an effect of the entire element shining, and this post will largely be based on that code.


CSS has the concept of an animation, to define it, use the following syntax:

.growOnHover:hover:after {
    animation: growAnimation 1s;

Here is the HTML referencing this:

<a href="#" class="growOnHover">Grow</a>

Tge “growAnimation” refers to a KeyFrame:

@keyframes growAnimation {
    from {width: 100px; height: 100px;}
    to {width: 110px; height: 110px;}

The effect

The effect that I want is for a light to run around the circumference of the button when it’s hovered over. In this case, instead of animating from .. to, we can specify at which stage a particular section of the animation kicks in.

.borderShine:after {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 5;
  height: 5;
  opacity: 0;  

  border-radius: 1;

  background: rgba(255, 255, 255, 10);

.borderShine:hover:after {
  animation: shineAnimation 2s 1;  

@keyframes shineAnimation {
  0%   {left: 0; top: 0; width: 2; height: 2; opacity: 0}
  10%  {width: 100; height: 2}  
  20%  {left: 98; top: 0; width: 2; height: 2}
  25%  {opacity: 1;}
  30%  {height: 100}
  40%  {left: 98; top: 98; height: 2}
  50%  {left: 0; top: 98; width: 100}
  55%  {opacity: 1;}
  60%  {left: 0; top: 98; width: 2; height: 2}
  70%  {left: 0; top: 0; width: 2; height: 100}
  80%  {left: 0; top: 0; width: 2; height: 2}
  100% {opacity: 0;}

There are a few useful things to remember here:

  • The animation is a transition between the state that the screen is currently in, and the state that you want it to be in; so, for example, the opacity set to 1 at 25% will cause the white bar to gradually appear over the steps between the two. The reason that I’ve set opacity twice here is to prevent it from transitioning back too soon.
  • All the figures above are absolute (as my buttons are 100 x 100).


CSS Overlaying Controls (absolute and relative positioning)

Having looked at CSS in the past, and thought that it’s probably something that people who are better at UI design that me should concern themselves with, I’ve recently been playing with it while looking at the new Dot Net Core web apps.

The problem that I’m looking at in this particular article is how to overlay one control on top of another. I have no doubt that there are dozens of possibilities; but the two specific ones that I’ll be focusing on are positioning absolute and relative.


The idea here is for a web-page that looks like this:

Target Layout


The HTML is pretty basic for this:

      <img src=""  />
    <div class="overlay">
      <input type="text" name="destination" />

CSS: absolute positioning

We have the basic elements, so now it’s down to CSS to make the screen above. By default, web browsers will render the div’s sequentially, and so the input box will appear below the image.

One possibility is to use “absolute” positioning. This means that I can position an element without regard to where other elements on the page might be; here’s an example:

.overlay {
    position: absolute; 
    top: 30%;         
    text-align: center;
    z-index: 10;

img {
  background-color: blue;
  width: 100%;
  height: 500px;

I’ve used 30% here because it matters on the size of your viewport – so that’s not ideal. Also, the centre align doesn’t work. This kind of makes sense when you think about it, because you’re using an absolute position – so what do you want to centralise it to?

CSS: relative positioning

Relative positioning took me a while to work out. It sounds like it’s relative to something else – but it’s actually relative to itself. Here’s what I tried for relative positioning:

.overlay {
    position: relative;
    width: 80%;
    height: 35px;
    top: -50px;
    border: none;
    text-align: center;
    z-index: 10;    

As you can see, it’s top position is negative, so it moves up from where it would have been. Also, because the positioning is relative, the centre align now works, because it’s back in the flow of the page.