twine-css-template VS animate.css

Second, I would recommend grabbing this stylesheet and copy-pasting it into the stylesheet section of your twine story. It's quite nicely annotated with comments throughout that will help you figure out what you can do in terms of CSS styling.

Harlowe can be a pain in the ass to do CSS for since it's pretty opaque about it. I'd suggest grabbing this template stylesheet and dropping it into your game, then messin' about with the values.

Animate.css

Animate.css (https://animate.style/)

Now, the content slide animation will come in later, as shown below: How about creating an altogether custom animation? Let’s make a bounce animation.

There are a few ways to create animations in React, but all of them fall into two broad categories: CSS animations, which change visual state by applying CSS rules; and, JavaScript animations, which use JavaScript to change the properties of the element. In either of those categories, you can either implement the animation from scratch, or you can use a library. On the CSS side, you can compose your animations with CSS rules, or you can use third-party libraries like Animate.css.

Use actual CSS/JS to make the effects. You can learn most of it if you search for CSS3 animations (Mozilla dev docs are a good option). You could also use a library like Animate.css

Animate.css: This CSS animation library offers different pre-built animations that can be easily applied to elements on a website. Different animations in the library, such as fades, slides, flips, and more, can be used to animate CSS Grid layouts.