Speedier tunes

I wrote a little while back about improving performance on The Session by reducing runtime JavaScript in favour of caching on the server. This is on the pages for tunes, where the SVGs for the sheetmusic are now inlined instead of being generated on the fly.

It worked. But I also wrote:

A page like that with lots of sheetmusic and plenty of comments is going to have a hefty page weight and a large DOM size. I’ve still got a fair bit of main-thread work happening, but now the bulk of it is style and layout, whereas previously I had the JavaScript overhead on top of that.

Take a tune like Out On The Ocean. It has 27 settings. That’s a lot of SVG markup that needs to be parsed, styled and rendered, even if it’s inline.

Then I remembered a very handy CSS property called content-visibility:

It enables the user agent to skip an element’s rendering work (including layout and painting) until it is needed — which makes the initial page load much faster.

Sounds great! But there are two gotchas.

The first gotcha is that if a browser doesn’t paint the element, it doesn’t know how much space the element should take up. So you need to provide dimensions. At the very least you need to provide a height value. Otherwise when the element comes into view and gets rendered, it pushes down on the content below it. You’d see a sudden jump in the scrollbar position.

The solution is to provide a value for contain-intrinsic-size. If your content is dynamic—from, say, a CMS—then you’re out of luck. You don’t know how long the content is.

Luckily, in my case, I could take a stab at it. I know how many lines of sheetmusic there are for each tune setting. Each line takes up roughly the same amount of space. If I multiply that amount of space by the number of lines then I’ve got a pretty good approximation of the height of the sheetmusic. I apply this with the contain-intrinsic-block-size property.

So each piece of sheetmusic has an inline style attribute with declarations like this:

content-visibility: auto;
contain-intrinsic-block-size: 380px;

It works a treat. I did a before-and-after check with pagespeed insights on the page for Out On The Ocean. The “style and layout” part of the main thread work went down considerably. Total blocking time went from more than 600 milliseconds to less than 400 milliseconds.

Not a bad result for a little bit of CSS!

I said there was a second gotcha. That’s browser support.

Right now content-visibility is only supported in Chrome and Edge. But that’s okay. This is a progressive enhancement. Adding this CSS has no detrimental effect on the browsers that don’t understand it (and when they do ship support for it, it’ll just start working). I’ve said it before and I’ll say it again: the forgiving error-parsing in HTML and CSS is a killer feature of the web. Browsers just ignore what they don’t understand. That’s what makes progressive enhancement like this possible.

And actually, there’s something you can do for all browsers. Even browsers that don’t support content-visibility still understand containment. So they’ll understand contain-intrinsic-size. Pair that with a contain declaration like this to tell the browser that this chunk of content isn’t going to reflow or get repainted:

contain: layout paint;

Here’s what MDN says about contain:

The contain CSS property indicates that an element and its contents are, as much as possible, independent from the rest of the document tree. Containment enables isolating a subsection of the DOM, providing performance benefits by limiting calculations of layout, style, paint, size, or any combination to a DOM subtree rather than the entire page.

So if you’ve got a chunk of static content, you might as well apply contain to it.

Again, not bad for a little bit of CSS!

abc to SVG | CSS-Tricks

Aw, this is so nice! Chris points to the way that The Session generates sheet music from abc text:

The SVG conversion is made possible entirely in JavaScript by an open source library. That’s the progressive enhancement part. Store and ship the basic format, and let the browser enhance the experience, if it can (it can).

Here’s another way of thinking of it: I was contacted by a blind user of The Session who hadn’t come across abc notation before. Once they realised how it worked, they said it was like having alt text for sheet music! 🤯

Pivoting From React to Native DOM APIs: A Real World Example - The New Stack

One dev team made the shift from React’s “overwhelming VDOM” to modern DOM APIs. They immediately saw speed and interaction improvements.

Yay! But:

…finding developers who know vanilla JavaScript and not just the frameworks was an “unexpected difficulty.”

Boo!

Also, if you have a similar story to tell about going cold turkey on React, you should share it with Richard:

If you or your company has also transitioned away from React and into a more web-native, HTML-first approach, please tag me on Mastodon or Threads. We’d love to share further case studies of these modern, dare I say post-React, approaches.

Faster Connectivity !== Faster Websites - Jim Nielsen’s Blog

The bar to overriding browser defaults should be way higher than it is.

Amen!

Standing still - a performance tinker | Trys Mudford

What Trys describes here mirrors my experience too—it really is worth occasionally taking a little time to catch the low-hanging fruit of your site’s web performance (and accessibility):

I’ve shaved nearly half a megabyte off the page size and improved the accessibility along the way. Not bad for an evening of tinkering.

What Is A Single-page Application?: HeydonWorks

You can’t create a complex modern web application like Google Mail without JavaScript and a SPA architecture. Google Mail is a webmail client and webmail clients existed some time before JavaScript became the language it is today or frameworks like Angular JS or Angular BS existed. However, you cannot create a complex modern web application like Google Mail without JavaScript. Google Mail itself offers a basic HTML version that works perfectly well without JavaScript of any form—let alone a 300KB bundle. But, still, you cannot create a complex modern web application like Google Mail without JavaScript. Just keep saying that. Keep repeating that line in perpetuity. Keep adding more and more JavaScript and calling it good.