Switch Function

The switch() function would provide conditional logic in CSS values, similar to the Sass if() function, but with access to some essential client-side values for comparison. The initial proposal resolves around available-inline-size, but that could be extended down the road.

Switches work off the built-in “phases” that a browser rendering-engine goes through. Some properties (like display) need to resolve early, while other properties (eg color) are not resolved until later. The switch function could make different queries available on different properties, based on their place in the rendering lifecycle.

Resources

From Brian Kardell & Igalia

Background

The @container block approach relies on CSS Containment and external-queries to avoid infinite loops. That approach would require single-axis size containment for most use-cases. Such 1D containment is still theoretical, and Safari/webkit do not yet support 2D containment.

The switch() proposal avoids those issues by limiting the queries that can be switched on different properties – ensuring e.g. available-inline-size cannot be queried until it has been resolved. Those limits come from the internal architecture of browser engines, and make it possible to implement switch() without relying on other hypothetical CSS features.

This might require teaching authors the essential phases of browser rendering, for a better understanding of which properties would support what queries. But that is similar to teaching the values of contain when using a container-based approach.

Prototype

Igalia has already developed a working prototype in Chromium.

There have been several proposals and a draft spec for inline conditional functions in CSS. Here are some highlights…

Nth-value

/* nth-value(<index>; <value>; <value> [; <value>]*) */
.card {
color: nth-value(var(--color-index, 1); maroon; tomato);
gap: nth-value(var(--size-index, 1); 1em; 2em);
flex-direction: nth-value(var(--size-index, 1); column; row);
}

/* change the value based on media/container sizes */
@media (prefers-color-scheme: dark) { html { --color-index: 2; } }
@container (min-width: 45em) { .card { --size-index: 2; } }

This can be used anywhere to toggle between a number of different values by changing the value of a custom property. It doesn’t provide any new features, but useful syntax sugar for quickly switching between inline values based on existing conditionals. Those values can all remain defined in a single place, rather than being split across at-rules.

Condition

Syntax

The initial proposal suggests:

.foo {
display: grid;
grid-template-columns: switch(
(available-inline-size > 1024px) 1fr 4fr 1fr;
(available-inline-size > 400px) 2fr 1fr;
(available-inline-size > 100px) 1fr;
default 1fr;
);
}

The implemented prototype is a bit different, but seems like a temporary way to fit existing CSS syntax:

.foo {
display: grid;
grid-template-columns: switch(
auto /
(available-inline-size > 1000px) 1fr 2fr 1fr 2fr /
(available-inline-size > 500px) auto 1fr /
);
}

There is also a comment from Fantasai with a proposal to make the syntax more efficient.

.foo {
grid-template-columns: switch( available-inline-size ?
(? > 1024px) 1fr 4fr 1fr;
(? > 400px) 2fr 1fr;
(? > 100px) 1fr;
1fr;
);
}

Thoughts

The two main “responsive component” approaches complement each other well – because they come at the same question (and all the tradeoffs) from opposite sides. The main advantages of switch() show up when you want to query the size of a grid-track rather than the element generating that track.

The switch() function is also very flexible, and could solve many other context-responsive styling issues – like adjusting <em> styles based on the inherited font-style, or toggling values based on a variable. That seems promising on multiple fronts.

Any inline-conditional syntax is bound to take up space, and add clutter to CSS. I’m not sure that’s avoidable, but it would be good to try and make the syntax as light-weight as reasonable.

That becomes more of an issue if this were the only way to write conditional CSS. Many languages provide both inline & block conditionals, for good reason – there are cases for each – and I think that might also be the way to go in CSS.

Questions

Can we improve the balance of readability/efficiency?

This question seems intertwined with a few other active discussions about more generic block & inline conditionals. Several of them are linked from this proposal by Lea Verou:

Higher level custom properties that control multiple declarations

What observations can we support, on what properties?

The primary focus is on available-inline-size used in grid-templates, with mention of future ideas like inherited-font-style. I’d love to flesh out that thinking a bit more.

Even if available-inline-size is enough for Level 1, we need to define where and how it can be used.

Can a single switch contain multiple observations?

The existing proposals & demos show available-inline-size as the only value being observed. If more observations are allowed, will the syntax support combining/mixing observations within a single switch function?