Pure CSS Slide-Down Animation

Sometimes something seems like it should be really easy, but it turns out to be extremely difficult. The case we'll be checking out today is creating a slide-down animation using purely CSS. What could be so hard about that, right? 

What Are We Talking About?

If you're not sure what I mean by a "slide-down animation," check out the slideDown method from jQuery. This function will take a hidden element and make it visible by increasing the element's height from 0 to whatever the height of the element should be. That shouldn't be too hard, right?

How You Might Try to Do It

Let's take a look at some quick ideas of how you might expect to be able to do this easily. If you simply go by what I said earlier, you'd just set the height to 0, and then to expand it, set the height to the number that shows the whole element (with a transition as well; this is assumed for all of them, otherwise it won't be an animation). Something like this:

Note: This is using :target and links to activate the animations, so the lnks aren't toggles. You'll need to go back in the frame's history or press "Rerun" to toggle them off. 

Note that it works perfectly! Problem solved right? Yes and no. This works perfectly if you know the height of the element, but what if you don't know the height? We'll need a generic animation that will work no matter what height the element is. This is especially true on a responsive site, where the height can change depending on screen size.

In this case, the seemingly obvious solutions is to just set the height to auto in the expanded version's styles.

Oh how I wish this worked. This would make everything so much easier, but sadly transitions only work on numeric values. Sure, auto does eventually compute to a numeric value, but that message doesn't seem to get to transition, so it just pops right open instead of doing a nice animation.

So what can we do? How about CSS Transforms? We'll set scaleY to zero and then set it to one when it should be expanded. Will that work?

When you use a transform, it doesn't actually change the amount of space it takes up, so there are big gaps between the links instead of making that area appear at the same time the element starts expanding. So this isn't working either. So how about we take a look at the method that I discovered.

The Answer

The answer is to set everything that can affect height (e.g. vertical padding, line-height, etc.) to 0 and then set it to the value you'd like when expanded. You need to be careful with this because there are lots of things that can affect height. For example, if there were multiple paragraphs of text inside the element, you'd need to adjust the margins between those paragraphs. If you have any images, you'll need to make sure the height and vertical margins around it are also set to 0. This works better than just using height since you're far more likely to know the values of each of these properties than you are to know the height of the expandable element.

Note: I also set the text color to toggle between transparent and black to give it a fading look. This is not necessary, but looks a little nicer. You can also accomplish this with opacity.

Conclusion

I've spent days trying to figure this out in the past. There just had to be a way! But aside from using JavaScript, which is also pretty complicated, I just couldn't figure it out for the longest time. Now that I have it figured out, I hope it can help you as well. God bless and happy coding!