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So, I understand how to perform both CSS3 transitions and animations. What is not clear, and I've googled, is when to use which.

For example, if I want to make a ball bounce, it is clear that animation is the way to go. I could provide keyframes and the browser would do the intermediates frames and I'll have a nice animation going.

However, there are cases when a said effect can be achieved either way. A simple and common example would be implement the facebook style sliding drawer menu:

This effect can be achieved through transitions like so:

.sf-page {

    -webkit-transition: -webkit-transform .2s ease-out;

}



.sf-page.out {

    -webkit-transform: translateX(240px);

}

http://jsfiddle.net/NwEGz/

Or, through animations like so:

.sf-page {

    -webkit-animation-duration: .4s;

    -webkit-transition-timing-function: ease-out;

}



.sf-page.in {

    -webkit-animation-name: sf-slidein;

    -webkit-transform: translate3d(0, 0, 0);

}



.sf-page.out {

    -webkit-animation-name: sf-slideout;

    -webkit-transform: translateX(240px);

}



@-webkit-keyframes sf-slideout {

    from { -webkit-transform: translate3d(0, 0, 0); }

    to { -webkit-transform: translate3d(240px, 0, 0); }

}



@-webkit-keyframes sf-slidein {

    from { -webkit-transform: translate3d(240px, 0, 0); }

    to { -webkit-transform: translate3d(0, 0, 0); }

}

http://jsfiddle.net/4Z5Mr/

With HTML that looks like so:

<div class="sf-container">

    <div class="sf-page in" id="content-container">

        <button type="button">Click Me</button>

    </div>

    <div class="sf-drawer">

    </div>

</div>

And, this accompanying jQuery script:

$("#content-container").click(function(){

    $("#content-container").toggleClass("out");

    // below is only required for css animation route

    $("#content-container").toggleClass("in");

});

What I'd like to understand is what are the pros and cons of these approaches.

1. One obvious difference is that animating is taking a whole lot more code.


2. Animation gives better flexibility. I can have different animation for sliding out and in


3. Is there something that can be said about performance. Do both take advantage of h/w acceleration?


4. Which is more modern and the way going forward


5. Anything else you could add?

It looks like you've got a handle on how to do them, just not when to do them.

A transition is an animation, just one that is performed between two distinct states - i.e. a start state and an end state. Like a drawer menu, the start state could be open and the end state could be closed, or vice versa.

If you want to perform something that does not specifically involve a start state and an end state, or you need more fine grain control over the keyframes in a transition, then you've got to use an animation.

I'll let the definitions speak for themselves (according to Merriam-Webster):

Transition: A movement, development, or evolution from one form, stage, or style to another

Animation: Endowed with life or the qualities of life; full of movement

The names appropriately fit their purposes in CSS

So, the example you gave should use transitions because it is only a change from one state to another

1. Animation takes a lot more code unless you're using the same transition over and over, in which case an animation would be better.



2. 
   

   You can have different effects for sliding in and out without an animation. Just have a different transition on both the original rule and the modified rule:

   
   
   

   .two-transitions {
   
       transition: all 50ms linear;
   
   }
   
   
   
   .two-transitions:hover {
   
       transition: all 800ms ease-out;
   
   }
   
   

   
   



3. Animations are just abstractions of transitions, so if the transition is hardware accelerated, the animation will be. It makes no difference.




4. Both are very modern.




5. My rule of thumb is if I use the same transition three times, it should probably be an animation. This is easier to maintain and alter in the future. But if you are only using it once, it is more typing to make the animation and maybe not worth it.

Animations are just that - a smooth behavior of set of properties. In other words it specifies what should happen to a set of element's properties. You define an animation and describe how this set of properties should behave during the animation process.

Transitions on the other side specify how a property (or properties) should perform their change. Each change. Setting a new value for certain property, be it with JavaScript or CSS, is always a transition, but by default it is not smooth. By setting transition in the css style you define different (smooth) way to perform these changes.

It can be said that transitions define a default animation that should be performed every time the specified property has changed.

A shorter answer, straight on point:

Transition:

1. Needs a triggering element (:hover, :focus etc.)


2. Only 2 animation states (start and end)


3. Used for simpler animations (buttons, dropdown menus and so on)


4. Easier to create but not so many animation/effect possibilities

Animation @keyframes:

1. It can be used for endless animations


2. Can set more than 2 states


3. No boundaries

Both use CPU acceleration for a much smoother effect.

Is there something that can be said about performance. Do both take
advantage of h/w acceleration?

In modern browsers, h/w acceleration occurs for the properties filter, opacity and transform. This is for both CSS Animations and CSS Transitions.

I believe CSS3 animation vs CSS3 transition will give you the answer you want.

Basically below are some takeaways :

1. If performance is a concern, then choose CSS3 transition.


2. If state is to be maintained after each transition, then choose CSS3 transition.


3. If the animation needs to be repeated, choose CSS3 animation. Because it supports animation-iteration-count.


4. If a complicated animation is desired. Then CSS3 animation is preferred.