🛸 Svelte CSS Image Slider: What is CSS :hover? #
First things first, what is a Svelte CSS Image Slider? The slider part is just an image gallery
with a strip of thumbnails. Click a thumbnail, and you see a large preview of the image you
selected. The hover part is making use of a some future CSS
We use some code by Jhey Thompkins here to produce that same effect when we hover over an image preview. Jhey added sprinkles; using
the future CSS
:has selector also to make the thumbnails either side
of the focussed one shift and grow, albeit not as much. Using just CSS to do this makes the transitions
:has is not supported in Firefox at the time of writing (although Chrome and Safari do support it).
Despite this, we can still grow and shift the focussed image in Firefox.
🏓 Svelte CSS Image Slider: What is a Bouncing Slider? #
The overscroll bouncing slider, is a little bonus for mobile device users. The hover effect doesn’t look great on smaller screens. For that reason, we replace it with a slider. So mobile visitors don’t miss out on the future CSS magic, we add the bounce effect to the scroller. This is similar to an effect you might see on an iPhone, if you try to scroll past the start or end. The scroll bounces back to the first or last image. We use some code Adam Argyle demoed at the CSS Day conference to get this effect, again only with CSS for a natural feel across devices.
🧱 Svelte CSS Image Slider: What are we Building? #
We will build the CSS image slider we just described with a few extras. A follower asked me to write a tutorial on creating a node SvelteKit app which we can run on Ubuntu or another Linux distribution. Taking that into account, I thought this would be a great opportunity to be able to use the Sharp image processing package on the server. Sharp requires a NodeJS environment to run, so is not well-suited to serverless deploys. For speed across all devices, we want to serve responsive image sets. It can be painful to produce the inputs manually as static assets for all devices.
Sharp comes to the rescue here. We run it on a Svelte server endpoint which resizes images to the required size on the fly. We will see, later, that we can add a public cache directive, so Cloudflare will be able to cache the generated images for us. This helps serve images efficiently, without creating static assets in all the various formats and sizes.
You can code along or check open the site in StackBlitz to have a play (GitHub repo link further down).
⚙️ Following Along? Get Started Here… #
Spin up a fresh SvelteKit app:
If this is your first time using SvelteKit, learn how to create a Svelte app.
Then, install these packages:
Be sure to install sharp in
dependencies and not
serverDependencies. See this GitHub issue for explanation of why server dependencies should not be added as
devDependencies when you use the Node adapter.
Svelte CSS Image Slider: Future CSS Setup #
To use some future CSS, we can set up PostCSS
postcss.config.cjs — click to expand code.
Then, update the Svelte Config file (
svelte.config.js — click to expand code.
Images Setup #
Then, add create a
src/lib/data folder and add the JSON image metadata
below to a
images.json file in the new directory.
src/lib/data/images.json — click to expand code.
You can get the images themselves from the GitHub repo. Save them to the paths shown below (within
src/assets folder). Here are the links:
Images are all from unsplash. See JSON file above for credits.
🍽 Layout: Self‑hosting new Figtree Font #
We’re trying out the new, free Figtree Google sans serif font by Erik Kennedy here, which we self-host. Self-hosting helps with optimization, resulting in one less origin for visitor browsers to connect to. This brings HTTP/2 multiplexing potential benefits too.
Besides the new font, there is nothing extraordinary in the layout file. The site will have a page
for each image with a URL like
For that reason, we create a
src/routes/[slug] dynamic route folder.
+layout.svelte file in this new folder with the following
src/routes/[slug]/+layout.svelte — click to expand code.
📸 Image Generation Server Endpoint #
Let’s take a look server side next. We mentioned briefly earlier that we will generate
responsive images on the fly. We will see, later, we can speed up page load but serving images
appropriate to the user device. Although this is a fantastic performance optimization, it can mean
generating quite a few resized images of different sizes for each actual image we want to include.
Consider the fact that we want to support devices with
340px wide screens,
as well as show images up to
1024px wide on desktops with larger displays.
On top, to support Retina and other high density displays, we also make images available in double
width too as well as in WebP and AVIF
Cloudflare Early Hints #
To speed things up, we will include a
rel=preload in the page header
and include a public caching header. This should encourage Cloudflare to use 103 Early hints as well as cache the images. Although things will be slow for the first visitor as images get generated,
subsequent visitors will the cached versions. As a further optimization, we will set the cache header
immutable. This instruction tells browsers and caching servers
that we never expect to update the images. How, then, do we update them? That’s where the
hash part of the file path comes in! If we change the file, we can expect the hash to change too, and
immutable directive on the old path will not matter.
src/routes/assets/[hash]/[filename]/server.ts — click to expand code.
In brief, we check for
format query strings in the request URL, so we can respond with the right next-gen format and size. The
Cache-Control header is most important here. This is where we request
Cloudflare cache for a very long time and not to check for updated images. Finally, Sharp does the
transformation magic, and we use streams to optimize serving the images.
🍸 Svelte CSS Image Slider Server Code #
Here, we use Vite JSON import to pull in image meta from a JSON file within the project. We return
hrefs for all the
images (thumbnails and large preview) here. We include the hash mentioned earlier in these values.
Finally, we generate a low quality Base64 placeholder for each image. We will set these as the
background image, so the browser shows them while waiting for the full resolution images to load.
This is another speed optimization.
src/routes/[slug]/+page.server.ts — click to expand code.
Also, worth a mention is the use of Vite glob imports in line
11. This finds all
files in the assets folder. Using a glob import instead of naming the files makes it easier to swap
out the images (we still need to update the JSON meta file though).
Server Image Utility Functions #
We will need a couple of utility functions to generate the Base64 low quality placeholders and also image hashes. Although the MD5 hashing algorithm is no longer considered suitable for cryptographic purposes, it is fine to use it here; an adversary has nothing to gain from finding hash collisions for our images. Instead of using the full hash, we truncate it to just 10 characters, which should be sufficient for use in identifying changes in the images.
We use a Remix convention here in naming the server image utilities. While with Remix the
.server portion means the code can only run on the server, as far as know, SvelteKit has no such guarantee
and the name is just to help us import the right files.
We use sharp here to generate the Base64 placeholders and the node Crypto module for hashes .
🧑🏽 Image Page Frontend Code #
This is the frontend template code for our page. However, the interesting image optimizations and CSS are in the two component files we look at in the next two sections.
src/routes/[slug]/+page.svelte — click to expand code.
🌄 Large Image Component #
This is the component for the large preview image. Possibly most important here is setting image heights and widths in the HTML template code to reduce Cumulative Layout
Shift . On top, we set
aspect-ratio in the CSS as well as set
height to auto.
We mention the image source set briefly before. Including this lets the browser choose the most
adequate image for the device. We decide on image densities of
2, needed for Retina displays. Then, we have image widths of
1024px. These correspond to image sizes required for a small
360px wide screen, the iPhone XR (widest common mobile) and the desktop breakpoints. We have utility functions
to generate the sets, and the final code will look something like this:
Image Component Code #
Remember, the query strings in the image URLs tell our server endpoint what size and format to serve. We opt for graceful degradation in the source sets, so start (optimistically) with AVIF, the newest and smallest next-gen format. If the browser does not support this, then we fall back to WebP and finally JPEG. For each set, the sizes attribute helps the browser decide which image to download while it is still working out the page layout. Although we could possibly cut down on the number of images here, we make the point that auto generation is a massive convenience.
Anyway, here is the component code (add it to
src/lib/components/Image.svelte — click to expand code.
As a final note here, the
rel=preload we include in the HTML
head is a way of preloading image source sets in Chromium .
fetchpriority is a newer priority hint, again supported by Chromium . This replaces the older
importance attribute. Finally, including
rel=preload link should instruct Cloudflare to cache the images
using the Early Hints API. As well as using Cloudflare as a CDN
Svelte CSS Image Slider: Client Image Utility Functions #
We have a few utility functions to help generate the image sets. Since the client code needs
these, they go in
src/lib/utilities/image.ts and not the
.server variant we created earlier.
src/lib/utilities/image.ts — click to expand code.
👍🏽 Thumbnails Component: Svelte Code #
This is where the exciting CSS is. We dissect it in the following sections. The markup is not too
different to the main image. You might notice we include stub bookend
<div aria-hidden="true" class="overscroller" />. These are used in Adam Argyle’s bouncy scroll, We’ll come to them later!
src/lib/components/Thumbnails.svelte with the following code:
🎬 Scroll into View Action #
This snippet uses Svelte actions — a more convenient Sveltey way of adding a scroll into view query selector on the
active image. Create a
src/lib/actions folder. In this new directory,
scrollCurrentIntoView.ts with this code:
src/lib/components/Thumbnails.svelte — click to expand code.
💅🏽 Thumbnails Component: CSS #
Here’s the CSS code in full, in case you are following along. We will just pick out the most
interesting bits to talk about in the rest of this section. You can paste the code at the end of
src/lib/components/Thumbnails.svelte — click to expand code.
We won’t go into detail on the
:hover steps effect here. Basically,
it is set using
lerp CSS custom properties. These are defined in the
src/routes/[slug]/+layout. Lerp stands for linear
interpolation and is a function which helps smooth motion in video games. Here Jhey uses it to for a natural , physical scaling on the focussed thumbnail and adjacent ones.
The main impact is on the flex-grow property , which fixes the relative sizes of the
.thumbnails elements. Tweak
none | [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ]
flex property is a recommended shorthand for
107 as well as the
--lerp custom properties in
src/routes/[slug]/+layout.css to get a feel for how it impacts the elements.
Bouncy slider overscroll #
Adam Argyle gave a marvellous talk at CSS day on scroll sliders . I hadn’t appreciated just how customizable they are. This is the most relevant CSS code for the effect:
overscroll-behavior-x: contain— this stops the whole window moving when you hit the end of the scroller,
scroll-snap-type: x mandatory— use this to snap scrolling at certain points (another nice alternative is
proximitywhich is less strict about things),
scroll-padding— adds padding to the scroll element,
scroll-snap-align— set on a thumbnail, controls whether the thumbnail is centre aligned or edge aligned when
scroll-snap-typeon scroller is
proximity. We use centre here, but set it to start and end for first and last elements,
scroll-snap-stop— fantastic for an image slider, stops you scrolling through dozens of thumbnails at once — make the viewer savour those thumbnails 😅.
.overscroller class is an Adam Argyle trick. Remember we added
bookend stub elements, to the slider? Those extend the slider slightly and the user exposes them when
they overscroll. However, we do not set
scroll-snap-align on them,
so when they do get exposed, the browser automatically snaps to the next snappable element,
which is the second or second-last element (or the first or last real thumbnail). The spring is provided
natively by the browser. Is that not neat? To allow for the stub elements, we set
end for the second
and penultimate element rather than first and last ones.
🗳 Poll #
🚧 Redirection #
A final detail, we redirect from the ‘
/’ route to the
first image using a server layout endpoint in
src/routes/+layout.server.ts — click to expand code.
🙌🏽 Svelte CSS Image Slider: Wrapping Up #
In this post, we saw how you can use sharp and future CSS to create a performant and feature-packed Svelte CSS image slider. In particular, we saw:
- some CSS code for :hover thumbnail effects,
- how to add bouncy overscroll sliders,
- quite a few optimizations for serving images.
Please clone the site from the Rodney Lab GitHub repo or open up the site on StackBlitz to try it out if you haven’t coded along. Hope you have found this post on creating a Svelte CSS image Slider useful! In a follow-up post, we will see how to deploy our node SvelteKit app to cloud hosting in a Linux box. We shall maintain the focus on performance as well as consider security there. I hope you will join me, and am also keen to hear what else you are doing with Svelte and ideas for future projects. Also let me know about any possible improvements to the content above.
🏁 Svelte CSS Image Slider: Summary #
Can you use future CSS with SvelteKit? #
- Yes, using PostCSS, you can start adding future CSS to your SvelteKit apps. We saw, once you install the PostCSS plugins, you need to add a `postcss.config.cjs` file to the project root folder. Then set `postcss: true` in your svelte.config.js config.preprocess options object. Finally, set the `lang=postcss` on your style tags in Svelte template files. That’s all you need!
How can you create a mobile bouncy overscroll effect in CSS? #
- We have seen Adam Argyle’s trick is a fantastic way to create a performant, widely supported CSS overscroll effect. The effect we are talking about lets the user scroll past the final elements, and pings back once they let go. This is common on iOS devices, but we can make it available across the board using CSS. First, we add extra stub elements either end. We can size them and using the `inline-width` property and style them to be transparent, to fit the container. Then, we set `scroll-snap-type` to mandatory on the scroller element. Finally, we set `scroll-snap-align`, to `start`, for example on all elements except the two stub ones. Now, when the user overscrolls, the scroller bounces to the first or last element.
🙏🏽 Svelte CSS Image Slider: Feedback #
If you have found this post useful, see links below for further related content on this site. I do hope you learned one new thing from the video. Let me know if there are any ways I can improve on it. I hope you will use the code or starter in your own projects. Be sure to share your work on Twitter, giving me a mention, so I can see what you did. Finally, be sure to let me know ideas for other short videos you would like to see. Read on to find ways to get in touch, further below. If you have found this post useful, even though you can only afford even a tiny contribution, please consider supporting me through Buy me a Coffee.
Just dropped the 1st part of a 2-part post on creating a node image slider with ♥️ SvelteKit.— Rodney (@askRodney) October 12, 2022
We use sharp to generate resized responsive images in next-gen formats and cache them in the CDN to make the page 🔥 fast.
Hope you find it useful.
Finally, feel free to share the post on your social media accounts for all your followers who will find it useful. As well as leaving a comment below, you can get in touch via @askRodney on Twitter and also askRodney on Telegram . Also, see further ways to get in touch with Rodney Lab. I post regularly on SvelteKit as well as Search Engine Optimization among other topics. Also, subscribe to the newsletter to keep up-to-date with our latest projects.