Tags: cascade

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Saturday, June 13th, 2020

CSS Custom Properties Fail Without Fallback · Matthias Ott – User Experience Designer

Matthias has a good solution for dealing with the behaviour of CSS custom properties I wrote about: first set your custom properties with the fallback and then use feature queries (@supports) to override those values.

Thursday, June 11th, 2020

CSS custom properties and the cascade

When I wrote about programming CSS to perform Sass colour functions I said this about the brilliant Lea Verou:

As so often happens when I’m reading something written by Lea—or seeing her give a talk—light bulbs started popping over my head (my usual response to Lea’s knowledge bombs is either “I didn’t know you could do that!” or “I never thought of doing that!”).

Well, it happened again. This time I was reading her post about hybrid positioning with CSS variables and max() . But the main topic of the post wasn’t the part that made go “Huh! I never knew that!”. Towards the end of her article she explained something about the way that browsers evaluate CSS custom properties:

The browser doesn’t know if your property value is valid until the variable is resolved, and by then it has already processed the cascade and has thrown away any potential fallbacks.

I’m used to being able to rely on the cascade. Let’s say I’m going to set a background colour on paragraphs:

p {
  background-color: red;
  background-color: color(display-p3 1 0 0);
}

First I’ve set a background colour using a good ol’ fashioned keyword, supported in browsers since day one. Then I declare the background colour using the new-fangled color() function which is supported in very few browsers. That’s okay though. I can confidently rely on the cascade to fall back to the earlier declaration. Paragraphs will still have a red background colour.

But if I store the background colour in a custom property, I can no longer rely on the cascade.

:root {
  --myvariable: color(display-p3 1 0 0);
}
p {
  background-color: red;
  background-color: var(--myvariable);
}

All I’ve done is swapped out the hard-coded color() value for a custom property but now the browser behaves differently. Instead of getting a red background colour, I get the browser default value. As Lea explains:

…it will make the property invalid at computed value time.

The spec says:

When this happens, the computed value of the property is either the property’s inherited value or its initial value depending on whether the property is inherited or not, respectively, as if the property’s value had been specified as the unset keyword.

So if a browser doesn’t understand the color() function, it’s as if I’ve said:

background-color: unset;

This took me by surprise. I’m so used to being able to rely on the cascade in CSS—it’s one of the most powerful and most useful features in this programming language. Could it be, I wondered, that the powers-that-be have violated the principle of least surprise in specifying this behaviour?

But a note in the spec explains further:

Note: The invalid at computed-value time concept exists because variables can’t “fail early” like other syntax errors can, so by the time the user agent realizes a property value is invalid, it’s already thrown away the other cascaded values.

Ah, right! So first of all browsers figure out the cascade and then they evaluate custom properties. If a custom property evaluates to gobbledygook, it’s too late to figure out what the cascade would’ve fallen back to.

Thinking about it, this makes total sense. Remember that CSS custom properties aren’t like Sass variables. They aren’t evaluated once and then set in stone. They’re more like let than const. They can be updated in real time. You can update them from JavaScript too. It’s entirely possible to update CSS custom properties rapidly in response to events like, say, the user scrolling or moving their mouse. If the browser had to recalculate the cascade every time a custom property didn’t evaluate correctly, I imagine it would be an enormous performance bottleneck.

So even though this behaviour surprised me at first, it makes sense on reflection.

I’ve probably done a terrible job explaining the behaviour here, so I’ve made a Codepen. Although that may also do an equally terrible job.

(Thanks to Amber for talking through this with me and encouraging me to blog about it. And thanks to Lea for expanding my mind. Again.)

Saturday, June 6th, 2020

Hybrid positioning with CSS variables and max() – Lea Verou

Yet another clever technique from Lea. But I’m also bookmarking this one because of something she points out about custom properties:

The browser doesn’t know if your property value is valid until the variable is resolved, and by then it has already processed the cascade and has thrown away any potential fallbacks.

That explains an issue I was seeing recently! I couldn’t understand why an older browser wasn’t getting the fallback I had declared earlier in the CSS. Turns out that custom properties mess with that expectation.

Friday, May 29th, 2020

CUBE CSS - Piccalilli

I really, really like Andy’s approach here:

The focus of the methodology is utilising the power of CSS and the web platform as a whole, with some added controls and structures that help to keep things a bit more maintainable and predictable. The end-goal is shipping as little CSS as possible—leaning heavily into progressive enhancement and modern techniques.

If you use the cascade for everything, you’re going to run into trouble. But equally, micro-managing styles on every element will also get you into trouble. I think Andy’s found a really great sweet spot here that gets the balance just right.

CUBE CSS in essence, is a progressive enhancement approach, vs a fight against the grain of CSS or a pixel-pushing your project to within an inch of its life approach.

Yes! It feels very “webby” to me.

Wednesday, January 22nd, 2020

The CSS Cascade

This is a wonderful interactive explanation of the way CSS hierarchy works—beautiful!

Thursday, December 5th, 2019

I <3 the cascade! | Go Make Things

Chris makes the valid observation that JavaScript programmers who bemoan the “global scope” of CSS are handily forgetting that JavaScript also has global scope by default.

JS is also global by default. We use IIFEs and wrapper functions to add scope.

And for all this talk about CSS being global, you can actually scope styles when you need to. It’s more-or-less the same way you do it in JavaScript.

Monday, June 10th, 2019

The CSS Mindset | Max Böck - Frontend Web Developer

This post absolutely nails what’s special about CSS …and why supersmart programmers might have trouble wrapping their head around it:

Other programming languages often work in controlled environments, like servers. They expect certain conditions to be true at all times, and can therefore be understood as concrete instructions as to how a program should execute.

CSS on the other hand works in a place that can never be fully controlled, so it has to be flexible by default.

Max goes on to encapsulate years of valuable CSS learnings into some short and snappy pieces of advices:

No matter what your level of CSS knowledge, this post has something for you—highly recommended!

Saturday, November 17th, 2018

Difference between currentColor & Custom Properties | Mike Riethmuller

I had to read through this twice, but I think I get it now (I’m not the sharpest knife in the drawer). Very useful if you’re doing theming in CSS.

Tuesday, August 21st, 2018

A Tale of Two Buttons

In defence of the cascade (especially now that we’ve got CSS custom properties).

I think embracing CSS’s cascade can be a great way to encourage consistency and simplicity in UIs. Rather than every new component being a free for all, it trains both designers and developers to think in terms of aligning with and re-using what they already have.

Remember, every time you set a property in CSS you are in fact overriding something (even if it’s just the default user agent styles). In other words, CSS code is mostly expressing exceptions to a default design.

Tuesday, June 12th, 2018

Resilient, Declarative, Contextual

This is really good breakdown of what’s different about CSS (compared to other languages).

These differences may feel foreign, but it’s these differences that make CSS so powerful. And it’s my suspicion that developers who embrace these things, and have fully internalized them, tend to be far more proficient in CSS.

Wednesday, November 23rd, 2016

CSS Inheritance, The Cascade And Global Scope: Your New Old Worst Best Friends – Smashing Magazine

A really terrific piece by Heydon that serves as a rousing defence of the cascade in CSS. It’s set up in opposition to methodologies like BEM (and there’s plenty of back’n’forth in the comments), but the truth is that every project is different so the more approaches you have in your toolkit, the better. For many projects, something like BEM is a good idea. For others, not so much.

Funnily enough, I’ve been working something recently where I’ve been embracing the approach that Heydon describes—although, to be fair, it’s a personal project where I don’t have to think about other developers touching the HTML or CSS.

Wednesday, April 27th, 2016

Sebastian Ly Serena

The greatest form ever made.