Journal tags: progress

The Session has been online in some form since the late 1990s. That’s long before web fonts existed.

To begin with, Times New Roman was the only game in town if you wanted serif type on a website. When Microsoft introduced Georgia it was a godsend. A beautiful typeface designed by Matthew Carter for the screen. I put it right at the start of my font stack for The Session.

Later, web fonts came along. Boy, does that short sentence belie the drama! There were very heated discussions about whether web browsers should provide this ability at all, and what it would mean for type foundries.

Microsoft led the way with their prorietary EOT format. Then everyone agreed on WOFF. Finally we got WOFF2, Electric Boogaloo.

Perhaps more important than that, we got intermediaries. Typekit, Fontdeck, and then the big daddy, Google Fonts.

That’s pretty much the state of play today. Oh yeah, and we’ve got variable fonts now.

I remember Nick Sherman presenting the idea of variable fonts at an Ampersand event years ago. I remember thinking “great idea, but it’ll never happen.” Pure science fiction. I thought the same thing when I first saw a conference presentation about a miraculous image format called Scalable Vector Graphics.

Sometimes I like to stop and take stock of what we take for granted in web browsers now. Web fonts. Variable web fonts. SVG. Flexbox. Grid. Media queries. Container queries. Fluid typography. And I haven’t even mentioned how we were once limited to just 216 colours on the web.

Georgia

Given all the advances in web typography, you might be wondering how my font strategy for The Session changed over the years.

It didn’t.

I mean, sure, I added fluid typography. That was a natural extension of my love for liquid layouts and, later, responsive design. But the font stack itself? That was still Georgia all the way.

Y’see, performance has always been a top priority for The Session. If I was going to replace a system font with a web font that the user had to download, it really needed to be worth it.

Over the years I dabbled with different typefaces but none of them felt quite right to me. And I still think Georgia is a beautiful typeface.

“But your website will look like lots of other websites!” some may cry. That used to be true when all we had was system fonts. But now that web fonts have become the norm, it’s actually pretty unusual to see Georgia in the wild.

Lora

Recently I found a font I liked. Part of why I like it is that it shares a lot of qualities with Georgia. It’s Lora by Olga Karpushina and Alexei Vanyashin.

I started to dabble with it and began seriously contemplating using it on The Session.

It’s a variable font, which is great. But actually, I’m not using that many weights on The Session. I could potentially just use a non-variable variety. It comes in fixed weights of regular, medium, semibold, and bold.

Alas, the regular weight (400) is a bit too light and the medium weight (500) is a bit too heavy. My goldilocks font weight is more like 450.

Okay, so the variable font it is. That also allows me to play around with some subtle variations in weights. As the font size gets bigger for headings, the font weight can reduce ever so slightly. And I can adjust the overall font weight down in dark mode (there’s no grading feature in this font, alas).

Subsetting

Lora supports a lot of alphabets, which is great—quite a few alphabets turn up on The Session occasionally. But this means that the font file size is quite large. 84K.

Subsetting to the rescue!

I created a subset of Lora that has everything except Cyrillic, Greek, and Latin Extended-B. I created another subset that only has Cyrillic, Greek, and Latin Extended-B. Now I’ve got two separate font files that are 48K and 41K in size.

I wrote two @font-face declarations for the two files. They’ve got the same font-family (Lora), the same font-weight (400 700), and the same font-style (normal) but they’ve got different values for unicode-range. That way, browsers know to only use appropriate file when characters on the page actually match the unicode range.

The first file is definitely going to be used. The second one might not even be needed on most pages.

I want to prioritise the loading of that first subsetted font file so it gets referenced in a link element with rel="preload".

The switcheroo

As well as file size, my other concern was how the swapping from Georgia to Lora would be perceived, especially on a slow connection. I wanted to avoid any visible rejiggering of the content.

This is where size-adjust comes in, along with its compadres ascent-override and descent-override.

Rather than adjusting the default size of Lora to match that of Georgia, I want to do it the other way around; adjust the fallback font to match the web font.

Here’s how I’m doing it:

@font-face {
    font-family: 'Fallback for Lora';
    src: local('Georgia');
    size-adjust: 105.77%;
    ascent-override: 95.11%;
    descent-override: 25.9%;
}

And then my font stack is:

font-family: Lora, 'Fallback for Lora', Georgia, serif;

It’s highly unlikely that any device out there has a system font called “Fallback for Lora” so I can be pretty confident that the @font-face adjustment rules will only get applied to browsers that have the right local font, Georgia.

But where did those magic numbers come from for size-adjust, ascent-override, and descent-override?

They came from Katie Hempenius. As well as maintaing a repo of font metrics, she provides the formula needed to calculate all three values. Or you could use this handy tool to eyeball it.

With that, Georgia gets swapped out for Lora with a minimum of layout shift.

First-timers and repeat visitors

Even with the layout shift taken care of, do I want to serve up web fonts to someone on a slow connection?

It depends. Specifically, it depends on whether it’s their first time visiting.

The Session already treats first time visitors differently to repeat visitors. The first time you visit the site, critical CSS is embedded in the head of the HTML page instead of being referenced in an external style sheet. Only once the page has loaded does the full style sheet also get downloaded and cached.

I decided that my @font-face rules pointing to the web fonts are not critical CSS. If it’s your first time visiting, those CSS rules only get downloaded after the page is done loading.

And unless you’re on a fast connection, you won’t see Georgia get swapped out for Lora. That’s because I’ve gone with a font-display value of “optional”.

Most people use “swap”. Some people use “fallback”. You’ve got to be pretty hardcore to use “optional”.

But the next page you go to, or the next time you come to the site, you more than likely will see Lora straight away. That’s because of the service worker I’ve got quietly putting static assets into the Cache API: CSS, JavaScript, and now web fonts.

So even though I’m prioritising snappy performance over visual consistency, it’s a trade-off that only really comes into play for first visits.

Next

I’m pretty happy with the overall strategy. Still, I’m not going to just set it and forget it. I’ll be monitoring the CRUX data for The Session keeping a particular eye on cumulative layout shift.

Before adding web fonts, the cumulative layout shift on The Session was zero. I think I’ve taken all the necessary steps to keep it nice and low, but if I’m wrong I’ll need to revisit my strategy.

Update: Big thanks to Roel Nieskens—of Wakamai Fondue fame—who managed to get the file size of my main subsetted font down even further; bedankt!

In my previous post, I mentioned that I’ve used the web install API in production. Specifically, I’ve used it on The Session. In order to do that, I had to register for the origin trial.

I’ve just signed up for another origin trial. This time it’s for the proposed focusgroup attribute:

The focusgroup HTML attribute is a proposed declarative way to add keyboard arrow-key navigation to composite widgets such as toolbars, tablists, menus, listboxes, etc. without writing any roving-tabindex JavaScript. One attribute replaces hundreds of lines of boilerplate.

I’ve got an HTML web component on The Session called tab-controls. And yes, there’s a bunch of code in there to listen for keyboard events and respond appropriately. I would very much like to rip that code out.

So now that I’ve opted into the origin trial, I’ve added this to my HTML:

<tab-controls role="tablist" focusgroup="tablist">

If this focusgroup attribute takes off, I’ll be able to remove the role attribute but for now, it’s very much needed.

In the JavaScript for my tab-controls custom element, I need to be able to detect support for focusgroup. Here’s how I’m doing it:

if (!this.focusgroup) {
// do all my key handling stuff here
}

Here’s the important thing: don’t use getAttribute('focusgroup') to test for browser support. That will return true if the attribute is in the HTML. But the attribute will only get converted into a property if the browser understands it.

Jake has a lot more detail on the differences between attributes and properties.

Anyway, I figured I’d share that little snippet in case you too were interested in trying out the focusgroup proposal using progressive enhancement.

I have websites in my dock on my computer. I have websites on the home screen of my phone. When I open these websites from the dock or from the home screen, they behave just like native apps. It’s brilliant!

But knowing that you can add a website to the dock or to the home screen remains arcane knowledge. If you don’t know it’s possible, most web browsers aren’t going to tell you it’s an option. As a site owner, you pretty much have to explain to your users what they can do.

Lately it feels like there’s been some movement to change this situation. Or, at the very least, there’s been some discussion.

As a site owner, what you want is a way for someone visiting your site to press a button to initiate the process of adding the site to the dock or the home screen.

From what I can see from the discussion, there are two contenders for how to do this: BeforeInstallPromptEvent versus navigator.install.

I’ve used both APIs in production, so I’d like to offer my balanced feedback on both:

• BeforeInstallPromptEvent sucks.
• navigator.install rocks.

To add more detail…

The BeforeInstallPromptEvent API relies on you capturing and delaying an event that may or not fire at all.

Based on some arbitrary heuristics, Chrome—for example—will prompt the user to install the current website. This easily-dismissable prompt looks indistinguishable from a prompt to sign up to a newsletter or grant permission for cookies, so most people dismiss it. The idea with BeforeInstallPromptEvent is that you capture that prompt, prevent it from prompting, and then release it when you think it’s an appropriate time.

If you think it takes mental gymnastics to understand that, just imagine what it’s like trying to implement it!

The whole thing rests on this flawed idea of an install prompt being shown when certain conditions are met. Other browser vendors rightly point out that users should be able to install any website they want. Ideally it should have a manifest file. But making a service worker a requirement is a step too far (and I say that as someone who literally wrote a book about service workers).

Contrast that with the Web Install API, AKA navigator.install.

Based on a user interaction—like a click on a button—the browser initiates the installation process. The user still has to confirm they want to do this, of course. You know how geolocation or web notifactions work? It’s like that. You can’t trigger any of those APIs without the user’s permission.

That’s it. No contest.

It would be absolutely wonderful if more browsers supported navigator.install. It would be a pain in the ass if they decided to support BeforeInstallPromptEvent instead.

Safari, Chrome, and Edge all allow you to install websites as though they’re apps.

On mobile Safari, this is done with the “Add to home screen” option that’s buried deep in the “share” menu, making it all but useless.

On the desktop, this is “Add to dock” in Safari, or “Install” in Chrome or Edge.

Firefox doesn’t offer this functionality, which as a shame. Firefox is my browser of choice but they decided a while back to completely abandon progressive web apps (though they might reverse that decision soon).

Anyway, being able to install websites as apps is fantastic! I’ve got a number of these “apps” in my dock: Mastodon, Bluesky, Instagram, The Session, Google Calendar, Google Meet. They all behave just like native apps. I can’t even tell which browser I used to initially install them.

If you’d like to prompt users to install your website as an app, there’s not much you can do other than show them how to do it. But that might be about to change…

I’ve been eagerly watching the proposal for a Web Install API. This would allow authors to put a button on a page that, when clicked, would trigger the installation process (the user would still need to confirm this, of course).

Right now it’s a JavaScript API called navigator.install, but there’s talk of having a declarative version too. Personally, I think this would be an ideal job for an invoker command. Making a whole new install element seems ludicrously over-engineered to me when button invoketarget="share" is right there.

Microsoft recently announced that they’d be testing the JavaScript API in an origin trial. I immediately signed up The Session for the trial. Then I updated the site to output the appropriate HTTP header.

You still need to mess around in the browser configs to test this locally. Go to edge://flags or chrome://flags/ and search for ‘Web App Installation API’, enable it and restart.

I’m now using this API on the homepage of The Session. Unsurprisingly, I’ve wrapped up the functionality into an HTML web component that I call button-install.

Here’s the code. You use it like this:

<button-install>
  <button>Install the app</button>
</button-install>

Use whatever text you like inside the button.

I wasn’t sure whether to keep the button element in the regular DOM or generate it in the Shadow DOM of the custom element. Seeing as the button requires JavaScript to do anything, the Shadow DOM option would make sense. As Tess put it, Shadow DOM is for hiding your shame—the bits of your interface that depend on JavaScript.

In the end I decided to stick with a regular button element within the custom element, but I take steps to remove it when it’s not necessary.

There’s a potential issue in having an element that could self-destruct if the browser doesn’t cut the mustard. There might be a flash of seeing the button before it gets removed. That could even cause a nasty layout shift.

So far I haven’t seen this problem myself but I should probably use something like Scott’s CSS in reverse: fade in the button with a little delay (during which time the button might end up getting removed anyway).

My connectedCallback method starts by finding the button nested in the custom element:

class ButtonInstall extends HTMLElement {
  connectedCallback () {
    this.button = this.querySelector('button');
    …
  }
customElements.define('button-install', ButtonInstall);

If the navigator.install method doesn’t exist, remove the button.

if (!navigator.install) {
  this.button.remove();
  return;
}

If the current display-mode is standalone, then the site has already been installed, so remove the button.

if (window.matchMedia('(display-mode: standalone)').matches) {
  this.button.remove();
  return;
}

As an extra measure, I could also use the display-mode media query in CSS to hide the button:

@media (display-mode: standalone) {
  button-install button {
    display: none;
  }
}

If the button has survived these tests, I can wire it up to the navigator.install method:

this.button.addEventListener('click', async (ev) => {
  await navigator.install();
});

That’s all I’m doing for now. I’m not doing any try/catch stuff to handle all the permutations of what might happen next. I just hand it over to the browser from there.

Feel free to use this code if you want. Adjust the code as needed. If your manifest file says display: fullscreen you’ll need to change the test in the JavaScript accordingly.

Oh, and make sure your site already has a manifest file that has an id field in it. That’s required for navigator.install to work.

Almost two months ago, I put out the call for speaker suggestions for Web Day Out. I got some good responses—thank you to everyone who took the time to get in touch.

The response that really piqued my interest was from Aleth Gueguen. She proposed a talk on progressive web apps, backed up with plenty of experience. The more I thought about it, the more I realised how perfect it would be for Web Day Out.

So I’m very pleased to announce that Aleth will be speaking at Web Day Out about progressive web apps from the trenches:

Find out about the most important capabilities in progressive web apps and how to put them to work.

I’m really excited about this line-up! This is going to be a day out that you won’t want to miss. Get your ticket for a mere £225+VAT if you haven’t already!

See you in Brighton on 12 March, 2026!

Tim recently gave a talk at Smashing Conference in New York called One Step Ahead. Based on the slides, it looks like it was an excellent talk.

Towards the end, there’s a slide that could be the tagline for Web Day Out:

Betting on the browser is our best chance at long-term success.

Most of the talk focuses on two technologies that you can add to any website with just a couple of lines of code: view transitions and speculation rules.

I’m using both of them on The Session and I can testify to their superpowers—super-snappy navigations with smooth animations.

Honestly, that takes care of 95% of the reasons for building a single-page app (the other 5% would be around managing state, which most sites—e-commerce, publishing, whatever—don’t need to bother with). Instead build a good ol’-fashioned website with pages of HTML linked together, then apply view transitions and speculation rules.

I mean, why wouldn’t you do that?

That’s not a rhetorical question. I’m genuinely interested in the reasons why people would reject a simple declarative solution in favour of the complexity of doing everything with a big JavaScript framework.

One reason might be browser support. After all, both view transitions and speculation rules are designed to be used as progressive enhancements, regardless of how many browsers happen to support them right now. If you want to attempt to have complete control, I understand why you might reach for the single-page app model, even if it means bloating the initial payload.

But think about that mindset for a second. Rather than reward the browsers that support modern features, you would instead be punishing them. You’d be treating every browser the same. Instead of taking advantage of the amazing features that some browsers have, you’d rather act as though they’re no different to legacy browsers.

I kind of understand the thinking behind that. You assume a level playing field by treating every browser as though they’re Internet Explorer. But what a waste! You ship tons of uneccesary code to perfectly capable browsers.

That could be the tagline for React.

I was messing about with some images on a website recently and while I was happy enough with the arrangement on large screens, I thought it would be better to have the images in a kind of carousel on smaller screens—a swipable gallery.

My old brain immediately thought this would be fairly complicated to do, but actually it’s ludicrously straightforward. Just stick this bit of CSS on the containing element inside a media query (or better yet, a container query):

display: flex;
overflow-x: auto;

That’s it.

Oh, and you can swap out overflow-x for overflow-inline if, like me, you’re a fan of logical properties. But support for that only just landed in Safari so I’d probably wait a little while before removing the old syntax.

Here’s an example using pictures of some of the lovely people who will be speaking at Web Day Out:

While you’re at it, add this:

overscroll-behavior-inline: contain;

Thats prevents the user accidentally triggering a backwards/forwards navigation when they’re swiping.

You could add some more little niceties like this, but you don’t have to:

scroll-snap-type: inline mandatory;
scroll-behavior: smooth;

And maybe this on the individual items:

scroll-snap-align: center;

You could progressively enhance even more with the new pseudo-elements like ::scroll-button() and ::scroll-marker for Chromium browsers.

Apart from that last bit, none of this is particularly new or groundbreaking. But it was a pleasant reminder for me that interactions that used to be complicated to implement are now very straightforward indeed.

Here’s another example that Ana Tudor brought up yesterday:

You have a section with a p on the left & an img on the right. How do you make the img height always be determined by the p with the tiniest bit of CSS? 😼

No changing the HTML structure in any way, no pseudos, no background declarations, no JS. Just a tiny bit of #CSS.

Old me would’ve said it can’t be done. But with a little bit of investigating, I found a nice straightforward solution:

section >  img {
  contain: size;
  place-self: stretch;
  object-fit: cover;
}

That’ll work whether the section has its display set to flex or grid.

There’s something very, very satisfying in finding a simple solution to something you thought would be complicated.

Honestly, I feel like web developers are constantly being gaslit into thinking that complex over-engineered solutions are the only option. When the discourse is being dominated by people invested in frameworks and libraries, all our default thinking will involve frameworks and libraries. That’s not good for users, and I don’t think it’s good for us either.

Of course, the trick is knowing that the simpler solution exists. The information probably isn’t going to fall in your lap—especially when the discourse is dominated by overly-complex JavaScript.

So get yourself a ticket for Web Day Out. It’s on Thursday, March 12th, 2026 right here in Brighton.

I guarantee you’ll hear about some magnificent techniques that will allow you to rip out plenty of complex code in favour of letting the browser do the work.

There was a time when you needed to make a native app in order to take advantage of specific technologies. That time has passed.

Now you can do all of these things on the web:

• push notifications,
• offline storage,
• camera access,
• and more.

Take a look at the home screen on your phone. Looking at the apps you’ve downloaded from an app store, ask yourself how many of them could’ve been web apps.

Social media apps, airline apps, shopping apps …none of them are using technologies that aren’t widely available on the web.

“But”, you might be thinking, “it feels different having a nice icon on my homescreen that launches a standalone app compared to navigating to a bookmark in my web browser.”

I agree! And you can do that with a web app. All it takes the addition of one manifest file that lists which icons and colours to use.

If that file exists for a website, then once the user adds the website to their homescreen it will behave just like native app.

Try it for yourself. Go to instagram.com in your mobile browser and it to your homescreen (on the iPhone, you get to the “add to home screen” option from the sharing icon—scroll down the list of options to find it).

See how it’s now an icon on your home screen just like all your other apps? Tap that icon to see how it launches just like a native app with no browser chrome around it.

This doesn’t just work on mobile. Desktop browsers like Chrome, Edge, and Safari also allow you to install web apps straight from the browser and into your dock.

About half of the icons in my dock are actually web apps and I honestly can’t tell which is which. Mastodon, Instagram, Google Calendar, Google Docs …I’m sure most of those services are available as downloadable desktop apps, but why would I bother doing that when I get exactly the same experience by adding the sites to my dock?

From a business perspective, it makes so much sense to build a web app (or simply turn your existing website into a web app with the addition of a manifest file). No need for separate iOS or Android developer teams. No need to play the waiting game with updates to your app in the app store—on the web, updates are instant.

You can even use an impressive-sounding marketing term for this approach: progressive web apps:

A Progressive Web App (PWA) is a web app that uses progressive enhancement to provide users with a more reliable experience, uses new capabilities to provide a more integrated experience, and can be installed. And, because it’s a web app, it can reach anyone, anywhere, on any device, all with a single codebase. Once installed, a PWA looks like any other app, specifically:

• It has an icon on the home screen, app launcher, launchpad, or start menu.
• It appears when you search for apps on the device.
• It opens in a standalone window, wholly separated from a browser’s user interface.
• It has access to higher levels of integration with the OS, for example, URL handling or title bar customization.
• It works offline.

But there’s still one thing that native apps do better than the web. If you want to be able to monitor and track users to an invasive degree, the web can’t compete with the capabilities of native apps. That’s why you’ll see so many websites on your mobile device that implore to install their app from the app store.

If that’s not a priority for you, then you can differentiate yourself from your competitors by offering your users a progressive web app. Instead of having links to Apple and Google’s app stores, you can link to a page on your own site with installation instructions.

I can guarantee you that users won’t be able to tell the difference between a native app they installed from an app store and a web app they’ve added to their home screen.

If you’re a front-end developer and you don’t read Chris Ferdinandi’s blog, you should change that right now. Add that RSS feed to your feed reader of choice!

Lately he’s been posting about some of the thinking behind his Kelp UI library. That includes some great nuggets of wisdom around HTML web components.

First of all, he pointed out that web components don’t need a constructor(). This was news to me. I thought custom elements had to include this incantation at the start:

constructor () {
  super();
}

But it turns that if all you’re doing is calling super(), you can omit the whole thing and it’ll be done for you.

I immediately refactored all the web components I’m using on The Session. While I was at it, I implemented Chris’s bulletproof web component loading.

Now technically, I don’t need to do this. I’m linking to my JavaScript at the bottom of every page so I know it’s going to load after the HTML. But I don’t like having that assumption baked into my code.

For any of my custom elements that reference other elements in the DOM—using, say, document.querySelector()—I updated the connectedCallback() method to use Chris’s technique.

It turned out that there weren’t that many of my custom elements that were doing that. Because HTML web components are wrapped around existing markup, the contents of the custom element are usually what matters (rather than other elements on the same page).

I guess that’s another unexpected benefit to HTML web components. Because they’ve already got their own bit of DOM inside them, you don’t need to worry about when you load your markup and when you load your JavaScript.

And no faffing about with the dark arts of the Shadow DOM either.

With the release of a new Salter Cane album I figured it was high time to update the design of the band’s website.

Here’s the old version for reference. As you can see, there’s a connection there in some of the design language. Even so, I decided to start completely from scratch.

I opened up a text editor and started writing HTML by hand. Same for the CSS. No templates. No build tools. No pipeline. Nothing. It was a blast!

And lest you think that sounds like a wasteful way of working, I pretty much had the website done in half a day.

Partly that’s because you can do so much with so little in CSS these days. Custom properties for colours, spacing, and fluid typography (thanks to Utopia). Logical properties. View transitions. None of this takes much time at all.

Because I was using custom properties, it was a breeze to add a dark mode with prefers-color-scheme. I think I might like the dark version more than the default.

The final stylesheet is pretty short. I didn’t bother with any resets. Browsers are pretty consistent with their default styles nowadays. As long as you’ve got some sensible settings on your body element, the cascade will take care of a lot.

There’s one little CSS trick I think is pretty clever…

The background image is this image. As you can see, it’s a rectangle that’s wider than it is tall. But the web pages are rectangles that are taller than they are wide.

So how I should I position the background image? Centred? Anchored to the top? Anchored to the bottom?

If you open up the website in Chrome (or Safari Technical Preview), you’ll see that the background image is anchored to the top. But if you scroll down you’ll see that the background image is now anchored to the bottom. The background position has changed somehow.

This isn’t just on the home page. On any page, no matter how tall it is, the background image is anchored to the top when the top of the document is in the viewport, and it’s anchored to the bottom when you reach the bottom of the document.

In the past, this kind of thing might’ve been possible with some clever JavaScript that measured the height of the document and updated the background position every time a scroll event is triggered.

But I didn’t need any JavaScript. This is a scroll-driven animation made with just a few lines of CSS.

@keyframes parallax {
    from {
        background-position: top center;
    }
    to {
        background-position: bottom center;
    }
}
@media (prefers-reduced-motion: no-preference) {
        html {
            animation: parallax auto ease;
            animation-timeline: scroll();
        }
    }
}

This works as a nice bit of progressive enhancement: by default the background image stays anchored to the top of the viewport, which is fine.

Once the site was ready, I spent a bit more time sweating some details, like the responsive images on the home page.

But the biggest performance challenge wasn’t something I had direct control over. There’s a Spotify embed on the home page. Ain’t no party like a third party.

I could put loading="lazy" on the iframe but in this case, it’s pretty close to the top of document so it’s still going to start loading at the same time as some of my first-party assets.

I decided to try a little JavaScript library called “lazysizes”. Normally this would ring alarm bells for me: solving a problem with third-party code by adding …more third-party code. But in this case, it really did the trick. The library is loading asynchronously (so it doesn’t interfere with the more important assets) and only then does it start populating the iframe.

This made a huge difference. The core web vitals went from being abysmal to being perfect.

I’m pretty pleased with how the new website turned out.

The Session has been online for over 20 years. When you maintain a site for that long, you don’t want to be relying on third parties—it’s only a matter of time until they’re no longer around.

Some third party APIs are unavoidable. The Session has maps for sessions and other events. When people add a new entry, they provide the address but then I need to get the latitude and longitude. So I have to use a third-party geocoding API.

My code is like a lesson in paranoia: I’ve built in the option to switch between multiple geocoding providers. When one of them inevitably starts enshittifying their service, I can quickly move on to another. It’s like having a “go bag” for geocoding.

Things are better on the client side. I’m using other people’s JavaScript libraries—like the brilliant abcjs—but at least I can self-host them.

I’m using Leaflet for embedding maps. It’s a great little library built on top of Open Street Map data.

A little while back I linked to a new project called OpenFreeMap. It’s a mapping provider where you even have the option of hosting the tiles yourself!

For now, I’m not self-hosting my map tiles (yet!), but I did want to switch to OpenFreeMap’s tiles. They’re vector-based rather than bitmap, so they’re lovely and crisp.

But there’s an issue.

I can use OpenFreeMap with Leaflet, but to do that I also have to use the MapLibre GL library. But whereas Leaflet is 148K of JavaScript, MapLibre GL is 800K! Yowzers!

That’s mahoosive by the standards of The Session’s performance budget. I’m not sure the loveliness of the vector maps is worth increasing the JavaScript payload by so much.

But this doesn’t have to be an either/or decision. I can use progressive enhancement to get the best of both worlds.

If you land straight on a map page on The Session for the first time, you’ll get the old-fashioned bitmap map tiles. There’s no MapLibre code.

But if you browse around The Session and then arrive on a map page, you’ll get the lovely vector maps.

Here’s what’s happening…

The maps are embedded using an HTML web component called embed-map. The fallback is a static image between the opening and closing tags. The web component then loads up Leaflet.

Here’s where the enhancement comes in. When the web component is initiated (in its connectedCallback method), it uses the Cache API to see if MapLibre has been stored in a cache. If it has, it loads that library:

caches.match('/path/to/maplibre-gl.js')
.then( responseFromCache => {
    if (responseFromCache) {
        // load maplibre-gl.js
    }
});

Then when it comes to drawing the map, I can check for the existence of the maplibreGL object. If it exists, I can use OpenFreeMap tiles. Otherwise I use the old Leaflet tiles.

But how does the MapLibre library end up in a cache? That’s thanks to the service worker script.

During the service worker’s install event, I give it a list of static files to cache: CSS, JavaScript, and so on. That includes third-party libraries like abcjs, Leaflet, and now MapLibre GL.

Crucially this caching happens off the main thread. It happens in the background and it won’t slow down the loading of whatever page is currently being displayed.

That’s it. If the service worker installation works as planned, you’ll get the nice new vector maps. If anything goes wrong, you’ll get the older version.

By the way, it’s always a good idea to use a service worker and the Cache API to store your JavaScript files. As you know, JavaScript is unduly expensive to performance; not only does the JavaScript file have to be downloaded, it then has to be parsed and compiled. But JavaScript stored in a cache during a service worker’s install event is already parsed and compiled.

I wrote a book about service workers. It’s called Going Offline. It was first published by A Book Apart in 2018. Now it’s available to read for free online.

If you want you can read the book as a PDF, an ePub, or .mobi, but I recommend reading it in your browser.

Needless to say the web book works offline. Once you go to goingoffline.adactio.com you can add it to the homescreen of your mobile device or add it to the dock on your Mac. After that, you won’t need a network connection.

The book is free to read. Properly free. Not the kind of “free” where you have to supply an email address first. Why would I make you go to the trouble of generating a burner email account?

The site has no analytics. No tracking. No third-party scripts of any kind whatsover. By complete coincidence, the site is fast. Funny that.

For the styling of this web book, I tweaked the stylesheet I used for HTML5 For Web Designers. I updated it a little bit to use logical properties, some fluid typography and view transitions.

In the process of converting the book to HTML, I got reaquainted with what I had written almost seven years ago. It was kind of fun to approach it afresh. I think it stands up pretty darn well.

Ethan wrote about his feelings when he put two of his books online, illustrated by that amazing photo that always gives me the feels:

I’ll miss those days, but I’m just glad these books are still here. They’re just different than they used to be. I suppose I am too.

Anyway, if you’re interested in making your website work offline, have a read of Going Offline. Enjoy!