Being able to animate the CSS width and height properties would be super useful. Unfortunately at the moment it’s a sure-fire way to get your browser to scream in agony. In this 5 minute tutorial we’ll explore using the transform property to simulate animating the width of an element.

Don’t Animate the Width and Height Properties

Browsers don’t like it when they have to calculate the positions and sizes of elements on the webpage. Most elements in some way impact the rendering other elements, they push siblings to another position, or when their size is changed a parent element also changes size. Modifying the dimensions of one element can therefore have lots of unforeseen consequences.

Changing the width and/or height of an element will trigger the browser to determine which other elements (children, siblings, or parents) are impacted by the change and how those elements should be updated. This process is called a reflow and it will be followed by a repaint.

These operations are very taxing on the CPU, so to keep a website fast we should avoid triggering them as much as possible.

Using Transforms Instead

To run performant animations we need to use the CSS transform or opacity properties. In this article we’ll focus on transform

The transform property instructs the GPU to make some last minute updates to the texture of an element before drawing it to the screen. These updates can for instance be rotating, moving, and scaling of an element.

You can compare the GPU texure of an element with a picture you took of the element with your mobile phone. This picture only consists of pixels, all other information about the element is gone.

The GPU only has to deal with the pixels that present the element. Because GPUs are very good at dealing with pixels, the transform operations are super fast.

This also has a downside. We can only manipulate the pixels, not the contents of the element. Remember we only have the picture of the element, all other information is gone.

You can see below how this difference impacts the border-radius property. In the transform example the radius is not redrawn (like in the width example), it is simply scaled. Redrawing would require a repaint, the GPU cannot do that, it only deals with pixels.

Original width: 100px transform: scaleX(.5)

.square { width : 200px ; height : 200px ; border-radius : 40px ; } .square-resized { width : 100px } .square-transformed { transform : scaleX ( .5 ) }

Animating the transform property is a million times faster than animating width , height , or any of the other properties that impact layout and will trigger a reflow

But if we animate it, the result will be that weirdly stretched shape. It might be fast, but it doesn’t look great. It always reminds me of those aliens in Duke Nukem 3D getting squished by a door.

Stretched corners

9-Slice Scaling To The Rescue

Before border-radius was a thing we had to create an image for each corner and/or edge of an element to “fake” border radius. Depending on the design that could yield up to 8 images for a single element. These images would be layed out like this:

1 2 3 4 5 6 7 8 9

This is called the 9-slice scaling method. Tis method allows you to scale the element and stretch image 2, 4, 6, and 8, while linking 1, 3, 7, and 9 to their respective corners using absolute positioning.

Now our corners aren’t stretched when scaled, see below:

1 2 3 4 5 6 7 8 9

Luckily we live in a day and age where browsers support border-radius , we no longer have to rely on this technique for drawing rounded corners.

We can however use this technique to resize elements using the transform property. For the sake of brevity this example will only explore scaling the width of the element.

We’ll start with a structure like this:

1 2 3

Let’s apply this to our square. We’ll need one container and three child elements to represent the square left (1), center (2), and right (3) parts.

<div class= "square" > <div class= "left" ></div> <div class= "center" ></div> <div class= "right" ></div> </div>

Let’s look at the CSS

/* children will be positioned relativly to this element */ .square { position : relative ; height : 100px ; } .left , .center , .right { position : absolute ; top : 0 ; bottom : 0 ; } .left { background : red } .center { background : yellow } .right { background : blue } /* we need room for a 20 pixel border radius (on one side) */ .left , .right { width : 20px } .left { border-radius : 20px 0 0 20px } .right { border-radius : 0 20px 20px 0 } /* child layout definitions */ .center { /* center needs to be 20 pixels from the left, so it doesn't overlap with the .left element */ left : 20px ; /* needs a width of 1 pixel, this causes scaleX(60) is equal to 60 pixels */ width : 1px ; transform : scaleX ( 60 ); /* we need to scale the texture from the left side for it to align correctly, default is center */ transform-origin : left ; } .right { /* we need to move the right element to align with the right side of the .center element 20px + 60px = 80px */ transform : translateX ( 80px ); }

The result is a nicely colored square.

I’ve colored to element parts to make them individually recognizeable.

Now for our next trick, we can animate the .center and .right elements of the square.

.center { animation : center-animate 1s ease infinite alternate ; } .right { animation : right-animate 1s ease infinite alternate ; } @keyframes right-animate { 0 % { transform : translateX ( 140px ) } 100 % { transform : translateX ( 20px ) } } @keyframes center-animate { 0 % { transform : scaleX ( 120 ) } 100 % { transform : scaleX ( 0 ) } }

Looks good to me, let’s set the background color back to black.

If you’re on Firefox, this should look great. If you’re using Safari or Chrome you might notice a slight flickering between the center and right part.

I’m not sure why this happens. It might be a problem with alpha blending OR rounding of pixels. Anyway. By slightly overlapping the center element with the right element we can resolve this render issue. We’ll make sure the element is at least 1 pixel wide and scales to 121 pixels.

@keyframes center-animate { 0 % { transform : scaleX ( 121 ) } 100 % { transform : scaleX ( 1 ) } }

Huray! We got ourselves an animatable non-squishy square!

We could take this further and make it more flexible by moving the animation to JavaScript, creating a web component, or by using CSS variables, but for the purpose of this article this example should suffice.

View a demo of end result on CodePen

Hold Your Horses

Before you run of and start using this technique everywhere, please note that:

We’re still scaling the center square over the X axis so background images applied to the square will be stretched.

A box-shadow can be done but is a bit tricky to pull off.

can be done but is a bit tricky to pull off. The square cannot cut off other elements, as in, overflow: hidden won’t work.

Still, with a bit of trickery, you can use this to create some awesome effects. The file items and the drop area of the FilePond file upload library all use this technique to smoothly animate their height.

Doka Image Editor used this technique on both the x and y axis to render and animate the white crop rectangle on top of the image. This has since been partly replaced with WebGL.

Conclusion

The default width and height CSS properties are (along with most other properties) not suitable for animation. They impact render performance too much because updating them triggers the browser to re-evaluate related element positions and sizes.

If we use the transform property instead we can performantly animate our square on the GPU. While super fast this also introduces some limitations because the GPU only deals with pixels.

By applying the 9-slice technique we can work around some of these limitations and create an element that we can animate in a performant way.