Advancements like high-performance compiled JavaScript and hardware-accelerated rendering of HTML5 and CSS3 in Internet Explorer 9 and 10 allow Web developers to create richer and richer experiences. Two related features,
CSS3 Transitions and CSS3 Animations,
give Web developers a declarative way to add personality to Web page interactions
easily.
This blog post describes these two features as implemented in IE10 in the
Windows Developer Preview and Windows 8
Metro style apps written in HTML.
Same Markup—Except for the Vendor Prefix
Like nearly all features new to IE9 and IE10, CSS3 Transitions and Animations are
standards-based features implemented with “same markup” interoperability in mind.
However, unlike features such as border-radius, box-shadow, and text-shadow, which
are part of stable W3C specifications, Transitions and Animations are still at a
specification stage where vendors prefix their implementations. Therefore, in this
case, “same markup” must be qualified as “same markup except for the vendor prefix.”
Many other writers have written about these features, including Rich Bradshaw’s excellent
article Using CSS3 Transitions, Transforms
and Animation. Many articles that discuss these features use only
the -webkit- prefix in their examples. Users wishing to experiment in other browsers
need to copy the example and change -webkit- to -ms-, -moz-, or -o-, as appropriate.
Bradshaw’s examples are an exception; they already work in IE10!
Transitions
The function of CSS3 Transitions is very straightforward: smoothly change the computed value
of a CSS property from its old value to its new value. Moreover, changes in value
resulting from changes in an element’s CSS class or pseudo-class also trigger transitions.
Consider the following markup:
img {
opacity: 1;
transition-property: opacity;
transition-duration: 2s;
transition-delay: 0s;
transition-timing-function: linear;
}
img:hover {
opacity: 0;
}
The effect of this is that when the user moves his or her mouse over the image, the image will fade out smoothly
over 2 seconds starting immediately, as illustrated below (I’ve added a dropped
shadow on a wrapping element to illustrate the endpoint).
Fading one image to nothing over 2 seconds
The transition properties that cause this to occur are:
- transition-property
– specifies which CSS properties are to be transitioned. The keyword “all” causes
all animatable
properties to transition when changed. The default value is “all.” - transition-duration
– the time, in seconds or milliseconds, of the transition, starting after the transition-delay.
The default value is zero, meaning that the transition is immediate. - transition-delay
– the time, in seconds or milliseconds, after the value is changed before the transition
starts. The time may be negative, in which case the transition starts part way through
its duration. The default value is zero. - transition-timing-function
– describes how the intermediate values of a transition are calculated. This allows
a transition to change speed over its duration. The underlying function is a
cubic Bézier curve; keywords match common functions. The default value is
the keyword “ease,” a function that starts fast and slows down toward the end.
Fading one image to nothing is a simple example. Let’s say we wanted to fade one
image to another, as illustrated below.
Fading from one image to another over 2 seconds
The following markup accomplishes this (except that vendor prefixes must precede
all the transition properties).
HTML Fragment
<div id="imageWrapper">
<img id="backImage" src="imageB.jpg" /><img id="frontImage" src="imageA.jpg" />
</div>
CSS Fragment
#imageWrapper {
display: inline-block;
width: 400px;
height: 267px;
box-shadow: 2px 2px 5px 0px gray;
position: relative;
}
#imageWrapper img {
width: 400px;
height: 267px;
position: absolute;
transition-property: opacity;
transition-duration: 2s;
transition-timing-function: linear;
}
#imageWrapper #frontImage, #imageWrapper:hover #backImage {
opacity: 1;
}
#imageWrapper:hover #frontImage, #imageWrapper #backImage {
opacity: 0;
}
Here’s a working version of the markup above:
Move your mouse over the image to fade it to another.
Simple transitions, such as the one above, are moderately easy to simulate in JavaScript.
The benefits of CSS Transitions are their declarative definitions, making them easier
than script, and they run—at least in IE10—asynchronously to the main processing
thread resulting in smoother transitions and sites that are more responsive.
An interactive demo of
CSS3 Transitions is available on the
IE Test Drive site. The demo works in all browsers that support CSS3 Transitions,
including IE10 in the
Windows Developer Preview.
Animations
CSS3 Animations are similar to CSS3 Transitions in that they smoothly animate a CSS
value over time. The differences are (a) how one specifies the properties to animation,
(b) how one triggers the animation and (c) the complexity of the animation possible.
You define animations using a CSS “keyframes” at-rule. A simple keyframes rule that
matches the fade out behavior of the transition above would be:
@keyframes fadeOut {
from {
opacity: 1;
}
to {
opacity: 0;
}
}
We apply this to our image with this CSS:
img {
animation-duration: 2s;
animation-delay: 0s;
animation-timing-function: linear;
animation-fill-mode: forwards;
}
img:hover {
animation-name: fadeOut;
}
Many of these properties are familiar from our discussion
of transitions. The new ones are:
-
animation-fill-mode – the “forwards” value of this property means to maintain
the “to” property values from the end of the animation going forward in time. The
default value of this property is “none,” which causes the properties to return
to their non-animated values at the end of the animation. (It’s possible to construct
the CSS above without using animation-fill-mode. Simply add “opacity: 0;” to the
img:hover rule to maintain the ending opacity at 0.) -
animation-name – setting the animation-name property triggers the animation.
When the animation-name property is set, the
animation-delay time starts counting down. When animation-delay reaches
zero, the animation begins and continues for the
animation-duration. The
animation-timing-function behaves the same as the transition-timing-function
described above.
The power of CSS3 Animations lies in the ability to specify multiple keyframes with
properties and intermediate values that don’t have to stay within the bounds of
the start and end values. In CSS3 Transitions, intermediate values always progress
from the start to the end; they will never go outside that range. Animations do
not have that restriction.
This makes it possible to program a “bounce” such as shown in the markup and example
below.
#bouncingImage {
width: 400px;
height: 267px;
box-shadow: 2px 2px 5px 0px gray;
animation-duration: 2s;
animation-timing-function: ease-in-out;
animation-fill-mode: forwards;
}
#bouncingImage:hover {
animation-name: zoomInBounce;
}
@keyframes zoomInBounce {
from {
transform: scale(1);
}
30% {
transform: scale(1.4);
}
40% {
transform: scale(1.15);
}
50% {
transform: scale(1.35);
}
60% {
transform: scale(1.2);
}
70% {
transform: scale(1.3);
}
80% {
transform: scale(1.225);
}
90% {
transform: scale(1.275);
}
to {
transform: scale(1.25);
}
}
Move your mouse over the image to zoom it with a bounce effect.
An interactive demo of
CSS3 Animations is available on the
IE Test Drive site. The demo works in all browsers that support CSS3 Animations,
including IE10 in the
Windows Developer Preview.
Building Your Site’s Personality
The two examples shared in this post are unlikely to be things you adopt on your
site. Yet, well-designed transitions and animations are becoming an expected part
of a modern Web experience. Windows 8 Metro style uses fluid and subtle animations
extensively to help users better understand their interactions with the system.
These literally make Windows 8 Metro style apps feel more responsive to touch.
We hope the examples here, the IE Test Drive demos, and the growing number of articles
and examples elsewhere on the Web help you explore this new technology and add personality
to your Web site.
—Ted Johnson, Lead Program Manager for Graphics, Internet Explorer