That is your full information to CSS cascade layers, a CSS function that enables us to outline express contained layers of specificity, in order that we have now full management over which types take precedence in a mission with out counting on specificity hacks or !necessary. This information is meant that can assist you absolutely perceive what cascade layers are for, how and why you would possibly select to make use of them, the present ranges of assist, and the syntax of how you employ them.
Desk of Contents
Fast instance
Introduction: what are cascade layers?
The place do layers match within the cascade?
!necessary origins, context, and layers are reversed!
Establishing a layer order
Syntax: Working with cascade layers
Use instances: When would I need to use cascade layers?
Check your data: Which model wins?
Debugging layer conflicts in browser developer instruments
Browser assist and fallbacks
Extra sources
Fast instance
/* set up a layer order up-front, from lowest to highest precedence */
@layer reset, defaults, patterns, elements, utilities, overrides;
/* import stylesheets right into a layer (dot syntax represents nesting) */
@import url(‘framework.css’) layer(elements.framework);
/* add types to layers */
@layer utilities {
/* excessive layer precedence, regardless of low specificity */
[data-color=’brand’] {
colour: var(–brand, rebeccapurple);
}
}
@layer defaults {
/* increased specificity, however decrease layer precedence */
a:any-link { colour: maroon; }
}
/* un-layered types have the very best precedence */
a {
colour: mediumvioletred;
}
Introduction: what are cascade layers?
CSS Cascade Layers are supposed to resolve difficult issues in CSS. Let’s check out the primary drawback and the way cascade layers purpose to resolve it.
Downside: Specificity conflicts escalate
Many people have been in conditions the place we need to override types from elsewhere in our code (or a third-party instrument), as a consequence of conflicting selectors. And through the years, authors have developed various “methodologies” and “finest practices” to keep away from these conditions — equivalent to “solely utilizing a single class” for all selectors. These guidelines are normally extra about avoiding the cascade, fairly than placing it to make use of.
Managing cascade conflicts and selector specificity has typically been thought of one of many tougher — or at the least extra complicated — elements of CSS. That could be partly as a result of few different languages depend on a cascade as their central function, nevertheless it’s additionally true that the unique cascade depends closely on heuristics (an educated-guess or assumption constructed into the code) fairly than offering direct and express management to net authors.
Selector specificity, for instance — our major interplay with the cascade — is predicated on the belief that extra narrowly focused types (like IDs which are solely used as soon as) are possible extra necessary than extra generic and reusable types (like lessons and attributes). That’s to say: how particular the selector is. That’s a very good guess, nevertheless it’s not a very dependable rule, and that causes some points:
It combines the act of choosing parts, with the act of prioritizing rule-sets.The best method to ‘repair’ a battle with specificity is to escalate the issue by including in any other case pointless selectors, or (gasp) throwing the !necessary hand-grenade.
.overly#highly effective .framework.widget {
colour: maroon;
}
.my-single_class { /* add some IDs to this ??? */
colour: rebeccapurple; /* add !necessary ??? */
}
Resolution: cascade layers present management
Cascade layers give CSS authors extra direct management over the cascade so we are able to construct extra deliberately cascading programs with out relying as a lot on heuristic assumptions which are tied to choice.
Utilizing the @layer at-rule and layered @imports, we are able to set up our personal layers of the cascade — constructing from low-priority types like resets and defaults, by themes, frameworks, and design programs, as much as highest-priority types, like elements, utilities, and overrides. Specificity remains to be utilized to conflicts inside every layer, however conflicts between layers are at all times resolved through the use of the higher-priority layer types.
@layer framework {
.overly#highly effective .framework.widget {
colour: maroon;
}
}
@layer web site {
.my-single_class {
colour: rebeccapurple;
}
}
These layers are ordered and grouped in order that they don’t escalate in the identical approach that specificity and significance can. Cascade layers aren’t cumulative like selectors. Including extra layers doesn’t make one thing extra necessary. They’re additionally not binary like significance — all of a sudden leaping to the highest of a stack — or numbered like z-index, the place we have now to guess a giant quantity (z-index: 9999999?). In actual fact, by default, layered types are much less necessary than un-layered types.
@layer defaults {
a:any-link { colour: maroon; }
}
/* un-layered types have the very best precedence */
a {
colour: mediumvioletred;
}
The place do layers match within the cascade?
The cascade is a sequence of steps (an algorithm) for resolving conflicts between types.
button { background: rebeccapurple !necessary; }
.warning { background: maroon; }
what colour background?
</button>
With the addition of cascade layers, these steps are:
Selector specificity is just one small a part of the cascade, nevertheless it’s additionally the step we work together with most, and is usually used to refer extra typically to general cascade precedence. Folks would possibly say that the !necessary flag or the model attribute “provides specificity” — a fast approach of expressing that the model turns into increased precedence within the cascade. Since cascade layers have been added instantly above specificity, it’s affordable to consider them in the same approach: one step extra highly effective than ID selectors.
Nevertheless, CSS Cascade Layers additionally make it extra important that we absolutely perceive the function of !necessary within the cascade — not simply as a instrument for “growing specificity” however as a system for balancing considerations.
!necessary origins, context, and layers are reversed!
As net authors, we frequently consider !necessary as a approach of accelerating specificity, to override inline types or extremely particular selectors. That works OK typically (when you’re OK with the escalation) nevertheless it leaves out the first goal of significance as a function within the general cascade.
Significance isn’t there to easily improve energy — however to steadiness the facility between varied competing considerations.
Necessary origins
All of it begins with origins, the place a method comes from within the net ecosystem. There are three primary origins in CSS:
The browser (or person agent)The person (typically by way of browser preferences)Net authors (that’s us!)
Browsers present readable defaults for all the weather, after which customers set their preferences, after which we (authors) present the supposed design for our net pages. So, by default, browsers have the bottom precedence, person preferences override the browser defaults, and we’re capable of override everybody.
However the creators of CSS had been very clear that we must always not even have the ultimate phrase:
If conflicts come up the person ought to have the final phrase, however one must also permit the creator to connect model hints.
— Håkon Lie (emphasis added)
So significance supplies a approach for the browser and customers to re-claim their precedence when it issues most. When the !necessary flag is added to a method, three new layers are created — and the order is reversed!
!necessary browser types (strongest)!necessary person preferences!necessary creator stylesnormal creator stylesnormal person preferencesnormal browser types (least highly effective)
For us, including !necessary doesn’t change a lot — our necessary types are fairly near our regular types — however for the browser and person it’s a really highly effective instrument for regaining management. Browser default model sheets embody various necessary types that it could be unattainable for us to override, equivalent to:
iframe:fullscreen {
/* iframes in full-screen mode do not present a border. */
border: none !necessary;
padding: unset !necessary;
}
Whereas many of the widespread browsers have made it tough to add precise person stylesheets, all of them provide person preferences: a graphic interface for establishing particular person types. In that interface, there’s at all times a checkbox obtainable for customers to decide on if a web site is allowed to override their preferences or not. This is similar as setting !necessary in a person stylesheet:
Necessary context
The identical primary logic is utilized to context within the cascade. By default, types from the host doc (mild DOM) override types from an embedded context (shadow DOM). Nevertheless, including !necessary reverses the order:
!necessary shadow context (strongest)!necessary host contextnormal host contextnormal shadow context (least highly effective)
Necessary types that come from inside a shadow context override necessary types outlined by the host doc. Right here’s an odd-bird customized factor with some types written within the factor template (shadow DOM), and a few types within the host web page (mild DOM) stylesheet:
Each colour declarations have regular significance, and so the host web page mediumvioletred takes precedence. However the font-family declarations are flagged !necessary, giving benefit to the shadow-context, the place fantasy is outlined.
Necessary layers
Cascade layers work the identical approach as each origins and context, with the necessary layers in reverse-order. The one distinction is that layers make that conduct way more noticeable.
As soon as we begin utilizing cascade layers, we are going to should be way more cautious and intentional about how we use !necessary. It’s now not a fast method to bounce to the highest of the priorities — however an built-in a part of our cascade layering; a approach for decrease layers to insist that a few of their types are important.
Since cascade layers are customizable, there’s no pre-defined order. However we are able to think about beginning with three layers:
utilities (strongest)componentsdefaults (least highly effective)
When types in these layers are marked as necessary, they’d generate three new, reversed necessary layers:
!necessary defaults (strongest)!necessary elements!necessary utilitiesnormal utilitiesnormal componentsnormal defaults (least highly effective)
On this instance, the colour is outlined by all three regular layers, and the utilities layer wins the battle, making use of the maroon colour, because the utilities layer has the next precedence in @layers. However discover that the text-decoration property is marked !necessary in each the defaults and elements layers, the place necessary defaults take precedence, making use of the underline declared by defaults:
Establishing a layer order
We are able to create any variety of layers and title them or group them in varied methods. However crucial factor to do is to ensure our layers are utilized in the correct order of precedence.
A single layer can be utilized a number of instances all through the codebase — cascade layers stack within the order they first seem. The primary layer encountered sits on the backside (least highly effective), and the final layer on the prime (strongest). However then, above that, un-layered types have the very best precedence:
@layer layer-1 { a { colour: pink; } }
@layer layer-2 { a { colour: orange; } }
@layer layer-3 { a { colour: yellow; } }
/* un-layered */ a { colour: inexperienced; }
un-layered types (strongest)layer-3layer-2layer-1 (least highly effective)
Then, as mentioned above, any necessary types are utilized in a reverse order:
@layer layer-1 { a { colour: pink !necessary; } }
@layer layer-2 { a { colour: orange !necessary; } }
@layer layer-3 { a { colour: yellow !necessary; } }
/* un-layered */ a { colour: inexperienced !necessary; }
!necessary layer-1 (strongest)!necessary layer-2!necessary layer-3!necessary un-layered stylesnormal un-layered stylesnormal layer-3normal layer-2normal layer-1 (least highly effective)
Layers can be grouped, permitting us to do extra sophisticated sorting of top-level and nested layers:
@layer layer-1 { a { colour: pink; } }
@layer layer-2 { a { colour: orange; } }
@layer layer-3 {
@layer sub-layer-1 { a { colour: yellow; } }
@layer sub-layer-2 { a { colour: inexperienced; } }
/* un-nested */ a { colour: blue; }
}
/* un-layered */ a { colour: indigo; }
un-layered types (strongest)layer-3layer-3 un-nestedlayer-3 sub-layer-2layer-3 sub-layer-1layer-2layer-1 (least highly effective)
Grouped layers at all times keep collectively within the remaining layer order (for instance, sub-layers of layer-3 will all be subsequent to one another), however this in any other case behaves the identical as if the listing was “flattened” — turning this right into a single six-item listing. When reversing !necessary layer order, your entire listing flattened is reversed.
However layers don’t need to be outlined as soon as in a single location. We give them names in order that layers might be outlined in a single place (to determine layer order), after which we are able to append types to them from anyplace:
/* describe the layer in a single place */
@layer my-layer;
/* append types to it from anyplace */
@layer my-layer { a { colour: pink; } }
We are able to even outline a complete ordered listing of layers in a single declaration:
@layer one, two, three, 4, 5, and so forth;
This makes it doable for the creator of a web site to have remaining say over the layer order. By offering a layer order up-front, earlier than any third social gathering code is imported, the order might be established and rearranged in a single place with out worrying about how layers are utilized in any third-party instrument.
Syntax: Working with cascade layers
Let’s check out the syntax!
Order-setting @layer statements
Since layers are stacked within the order they’re outlined, it’s necessary that we have now a instrument for establishing that order multi functional place!
We are able to use @layer statements to do this. The syntax is:
@layer <layer-name>#;
That hash (#) means we are able to add as many layer names as we wish in a comma-separated listing:
@layer reset, defaults, framework, elements, utilities;
That may set up the layer order:
un-layered types (strongest)utilitiescomponentsframeworkdefaultsreset (least highly effective)
We are able to do that as many instances as we wish, however bear in mind: what issues is the order every title first seems. So it will have the identical consequence:
@layer reset, defaults, framework;
@layer elements, defaults, framework, reset, utilities;
The ordering logic will ignore the order of reset, defaults, and framework within the second @layer rule as a result of these layers have already been established. This @layer listing syntax doesn’t add any particular magic to the layer ordering logic: layers are stacked primarily based on the order through which the layers first seem in your code. On this case, reset seems first within the first @layer listing. Any @layer assertion that comes later can solely append layer names to the listing, however can’t transfer layers that exist already. This ensures that you would be able to at all times management the ultimate general layer order from one location — on the very begin of your types.
These layer-ordering statements are allowed on the prime of a stylesheet, earlier than the @import rule (however not between imports). We extremely advocate utilizing this function to determine all of your layers up-front in a single place so that you at all times know the place to look or make adjustments.
Block @layer guidelines
The block model of the @layer rule solely takes a single layer title, however then means that you can add types to that layer:
@layer <layer-name> {
/* types added to the layer */
}
You may put most issues inside an @layer block — media queries, selectors and types, assist queries, and so forth. The one issues you’ll be able to’t put inside a layer block are issues like charset, imports, and namespaces. However don’t fear, there’s a syntax for importing types right into a layer.
If the layer title hasn’t been established earlier than, this layer rule will add it to the layer order. But when the title has been established, this lets you add types to present layers from anyplace within the doc — with out altering the precedence of every layer.
If we’ve established our layer-order up-front with the layer assertion rule, we now not want to fret in regards to the order of those layer blocks:
/* set up the order up-front */
@layer defaults, elements, utilities;
/* add types to layers in any order */
@layer utilities {
[hidden] { show: none; }
}
/* utilities will override defaults, primarily based on established order */
@layer defaults {
* { box-sizing: border-box; }
img { show: block; }
}
Grouping (nested) layers
Layers might be grouped, by nesting layer guidelines:
@layer one {
/* sorting the sub-layers */
@layer two, three;
/* types … */
@layer three { /* types … */ }
@layer two { /* types … */ }
}
This generates grouped layers that may be represented by becoming a member of the father or mother and youngster names with a interval. Meaning the ensuing sub-layers can be accessed instantly from exterior the group:
/* sorting nested layers instantly */
@layer one.two, one.three;
/* including to nested layers instantly */
@layer one.three { /* … */ }
@layer one.two { /* … */ }
The foundations of layer-ordering apply at every degree of nesting. Any types that aren’t additional nested are thought of “un-layered” in that context, and have precedence over additional nested types:
@layer defaults {
/* un-layered defaults (increased precedence) */
:any-link { colour: rebeccapurple; }
/* layered defaults (decrease precedence) */
@layer reset {
a[href] { colour: blue; }
}
}
Grouped layers are additionally contained inside their father or mother, in order that the layer order doesn’t intermix throughout teams. On this instance, the highest degree layers are sorted first, after which the layers are sorted inside every group:
@layer reset.kind, default.kind, reset.media, default.media;
Leading to a layer order of:
un-layered (strongest)default groupdefault un-layereddefault.mediadefault.typereset groupreset un-layeredreset.mediareset.kind
Observe that layer names are additionally scoped in order that they don’t work together or battle with similarly-named layers exterior their nested context. Each teams can have distinct media sub-layers.
This grouping turns into particularly necessary when utilizing @import or <hyperlink> to layer whole stylesheets. A 3rd-party instrument, like Bootstrap, may use layers internally — however we are able to nest these layers right into a shared bootstrap layer-group on import, to keep away from potential layer-naming conflicts.
Layering whole stylesheets with @import or <hyperlink>
Total stylesheets might be added to a layer utilizing the brand new layer() operate syntax with @import guidelines:
/* types imported into to the <layer-name> layer */
@import url(‘instance.css’) layer(<layer-name>);
There’s additionally a proposal so as to add a layer attribute within the HTML <hyperlink> factor — though that is nonetheless beneath growth, and not but supported anyplace. This can be utilized to import third-party instruments or element libraries, whereas grouping any inner layers collectively beneath a single layer title — or as a approach of organizing layers into distinct recordsdata.
Nameless (un-named) layers
Layer names are useful as they permit us to entry the identical layer from a number of locations for sorting or combining layer blocks — however they don’t seem to be required.
It’s doable to create nameless (un-named) layers utilizing the block layer rule:
@layer { /* … */ }
@layer { /* … */ }
Or utilizing the import syntax, with a layer key phrase instead of the layer() operate:
/* types imported into to a brand new nameless layer */
@import url(‘../instance.css’) layer;
Every nameless layer is exclusive, and added to the layer order the place it’s encountered. Nameless layers can’t be referenced from different layer guidelines for sorting or appending extra types.
These ought to in all probability be used sparingly, however there could be a couple of use instances:
Initiatives may make sure that all types for a given layer are required to be positioned in a single place.Third-party instruments may “disguise” their inner layering inside nameless layers in order that they don’t turn out to be a part of the instrument’s public API.
Reverting values to the earlier layer
There are a number of ways in which we are able to use to “revert” a method within the cascade to a earlier worth, outlined by a decrease precedence origin or layer. That features various present world CSS values, and a brand new revert-layer key phrase that will even be world (works on any property).
Context: Present world cascade key phrases*
CSS has a number of world key phrases which can be utilized on any property to assist roll-back the cascade in varied methods.
preliminary units a property to the specified worth earlier than any types (together with browser defaults) are utilized. This may be shocking as we frequently consider browser types because the preliminary worth — however, for instance, the preliminary worth of show is inline, it doesn’t matter what factor we apply it to.inherit units the property to use a worth from its father or mother factor. That is the default for inherited properties, however can nonetheless be used to take away a earlier worth.unset acts as if merely eradicating all earlier values — in order that inherited properties as soon as once more inherit, whereas non-inherited properties return to their preliminary worth.revert solely removes values that we’ve utilized within the creator origin (i.e. the location types). That is what we wish typically, because it permits the browser and person types to stay intact.
New: the revert-layer key phrase
Cascade layers add a brand new world revert-layer key phrase. It really works the identical as revert, however solely removes values that we’ve utilized within the present cascade layer. We are able to use that to roll again the cascade, and use no matter worth was outlined within the earlier layers.
On this instance, the no-theme class removes any values set within the theme layer.
@layer default {
a { colour: maroon; }
}
@layer theme {
a { colour: var(–brand-primary, purple); }
.no-theme {
colour: revert-layer;
}
}
So a hyperlink tag with the .no-theme class will roll again to make use of the worth set within the default layer. When revert-layer is utilized in un-layered types, it behaves the identical as revert — rolling again to the earlier origin.
Reverting necessary layers
Issues get attention-grabbing if we add !necessary to the revert-layer key phrase. As a result of every layer has two distinct “regular” and “necessary” positions within the cascade, this doesn’t merely change the precedence of the declaration — it adjustments what layers are reverted.
Let’s assume we have now three layers outlined, in a layer stack that appears like this:
utilities (strongest)componentsdefaults (least highly effective)
We are able to flesh that out to incorporate not simply regular and necessary positions of every layer, but additionally un-layered types, and animations:
!necessary defaults (strongest)!necessary elements!necessary utilities!necessary un-layered stylesCSS animationsnormal un-layered stylesnormal utilitiesnormal componentsnormal defaults (least highly effective)
Now, after we use revert-layer in a standard layer (let’s use utilities) the result’s pretty direct. We revert solely that layer, whereas every thing else applies usually:
✅ !necessary defaults (strongest)✅ !necessary elements✅ !necessary utilities✅ !necessary un-layered types✅ CSS animations✅ regular un-layered types❌ regular utilities✅ regular elements✅ regular defaults (least highly effective)
However after we transfer that revert-layer into the necessary place, we revert each the traditional and necessary variations together with every thing in-between:
✅ !necessary defaults (strongest)✅ !necessary elements❌ !necessary utilities❌ !necessary un-layered types❌ CSS animations❌ regular un-layered types❌ regular utilities✅ regular elements✅ regular defaults (least highly effective)
Use instances: When would I need to use cascade layers?
So what kind of conditions would possibly we discover ourselves utilizing cascade layers? Listed here are a number of examples of when cascade layers make lots of sense, in addition to others the place they don’t make quite a bit sense.
Much less intrusive resets and defaults
One of many clearest preliminary use instances could be to make low-priority defaults which are simple to override.
Some resets have been doing this already by making use of the :when() pseudo-class round every selector. :when() removes all specificity from the selectors it’s utilized to, which has the essential affect desired, but additionally some downsides:
It needs to be utilized to every selector individuallyConflicts contained in the reset need to be resolved with out specificity
Layers permit us to extra merely wrap your entire reset stylesheet, both utilizing the block @layer rule:
/* reset.css */
@layer reset {
/* all reset types in right here */
}
Or while you import the reset:
/* reset.css */
@import url(reset.css) layer(reset);
Or each! Layers might be nested with out altering their precedence. This fashion, you need to use a third-party reset, and guarantee it will get added to the layer you need whether or not or not the reset stylesheet itself is written utilizing layers internally.
Since layered types have a decrease precedence than default “un-layered” types, this can be a good method to begin utilizing cascade layers with out re-writing your whole CSS codebase.
The reset selectors nonetheless have specificity info to assist resolve inner conflicts, with out wrapping every particular person selector — however you additionally get the specified final result of a reset stylesheet that’s simple to override.
Managing a posh CSS structure
As tasks turn out to be bigger and extra complicated, it may be helpful to outline clearer boundaries for naming and organizing CSS code. However the extra CSS we have now, the extra potential we have now for conflicts — particularly from totally different elements of a system like a “theme” or a “element library” or a set of “utility lessons.”
Not solely do we wish these organized by operate, nevertheless it can be helpful to arrange them primarily based on what elements of the system take precedence within the case of a battle. Harry Robert’s Inverted Triangle CSS does a very good job visualizing what these layers would possibly include.
In actual fact, the preliminary pitch for including layers to the CSS cascade used the ITCSS methodology as a major instance, and a information for growing the function.
There isn’t any specific method required for this, nevertheless it’s possible useful to limit tasks to a pre-defined set of top-level layers after which prolong that set with nested layers as acceptable.
For instance:
low degree reset and normalization typesfactor defaults, for primary typography and legibilitythemes, like mild and darkish modesre-usable patterns that may seem throughout a number of elementslayouts and bigger web page structuresindividual elementsoverrides and utilities
We are able to create that top-level layer stack on the very begin of our CSS, with a single layer assertion:
@layer
reset,
default,
themes,
patterns,
layouts,
elements,
utilities;
The precise layers wanted, and the way you title these layers, would possibly change from one mission to the following.
From there, we create much more detailed layer breakdowns. Perhaps our elements themselves have defaults, buildings, themes, and utilities internally.
@layer elements {
@layer defaults, buildings, themes, utilities;
}
With out altering the top-level construction, we now have a method to additional layer the types inside every element.
Utilizing third-party instruments and frameworks
Integrating third-party CSS with a mission is among the commonest locations to run into cascade points. Whether or not we’re utilizing a shared reset like Normalizer or CSS Treatment, a generic design system like Materials Design, a framework like Bootstrap, or a utility toolkit like Tailwind — we are able to’t at all times management the selector specificity or significance of all of the CSS getting used on our websites. Generally, this even extends to inner libraries, design programs, and instruments managed elsewhere in a company.
Because of this, we frequently need to construction our inner CSS across the third-party code, or escalate conflicts after they come up — with artificially excessive specificity or !necessary flags. After which we have now to take care of these hacks over time, adapting to upstream adjustments.
Cascade layers give us a method to slot third-party code into the cascade of any mission precisely the place we wish it to stay — irrespective of how selectors are written internally. Relying on the kind of library we’re utilizing, we’d try this in varied methods. Let’s begin with a primary layer-stack, working our approach up from resets to utilities:
@layer reset, kind, theme, elements, utilities;
After which we are able to incorporate some instruments…
Utilizing a reset
If we’re utilizing a instrument like CSS Treatment, we’d even have some reset types of our personal that we need to embody. Let’s import CSS Treatment right into a sub-layer of reset:
@import url(‘treatment.css’) layer(reset.treatment);
Now we are able to add our personal reset types to the reset layer, with none additional nesting (except we wish it). Since types instantly in reset will override any additional nested types, we might be positive our types will at all times take precedence over CSS Treatment if there’s a battle — it doesn’t matter what adjustments in a brand new launch:
@import url(‘treatment.css’) layer(reset.treatment);
@layer reset {
:is(ol, ul)[role=’list’] {
list-style: none;
padding-inline-start: 0;
}
}
And because the reset layer is on the backside of the stack, the remainder of the CSS in our system will override each Treatment, and our personal native reset additions.
Utilizing utility lessons
On the different finish of our stack, “utility lessons” in CSS generally is a helpful method to reproduce frequent patterns (like extra context for display readers) in a broadly-applicable approach. Utilities have a tendency to interrupt the specificity heuristic, since we wish them outlined broadly (leading to a low specificity), however we additionally typically need them to “win” conflicts.
By having a utilities layer on the prime of our layer stack, we are able to make that doable. We are able to use that in the same method to the reset instance, each loading exterior utilities right into a sub-layer, and offering our personal:
@import url(‘tailwind.css’) layer(utilities.tailwind);
@layer utilities {
/* from https://kittygiraudel.com/snippets/sr-only-class/ */
/* however with !necessary faraway from the properties */
.sr-only {
border: 0;
clip: rect(1px, 1px, 1px, 1px);
-webkit-clip-path: inset(50%);
clip-path: inset(50%);
top: 1px;
overflow: hidden;
margin: -1px;
padding: 0;
place: absolute;
width: 1px;
white-space: nowrap;
}
}
Utilizing design programs and element libraries
There are lots of CSS instruments that fall someplace in the midst of our layer stack — combining typography defaults, themes, elements, and different elements of a system.
Relying on the actual instrument, we’d do one thing much like the reset and utility examples above — however there are a couple of different choices. A extremely built-in instrument would possibly deserve a top-level layer:
@layer reset, bootstrap, utilities;
@import url(‘bootstrap.css’) layer(bootstrap);
If these instruments begin to present layers as a part of their public API, we may additionally break it down into elements — permitting us to intersperse our code with the library:
@import url(‘bootstrap/reset.css’) layer(reset.bootstrap);
@import url(‘bootstrap/theme.css’) layer(theme.bootstrap);
@import url(‘bootstrap/elements.css’) layer(elements.bootstrap);
@layer theme.native {
/* types right here will override theme.bootstrap */
/* however not intervene with types from elements.bootstrap */
}
Utilizing layers with present (un-layered, !important-filled) frameworks
As with all main language change, there’s going to be an adjustment interval when CSS Cascade Layers turn out to be extensively adopted. What occurs in case your group is able to begin utilizing layers subsequent month, however your favourite framework decides to attend one other three years earlier than they swap over to layered types? Many frameworks will possible nonetheless use !necessary extra typically than we’d like! With !necessary layers reversed, that’s not preferrred.
Nonetheless, layers can nonetheless assist us remedy the issue. We simply need to get intelligent about it. We resolve what layers we wish for our mission, and which means we are able to add layers above and additionally under the framework layers we create.
For now, although, we are able to use a decrease layer to override !necessary types from the framework, and the next layer to override regular types. One thing like this:
@layer framework.necessary, framework.bootstrap, framework.native;
@import url(‘bootstrap.css’) layer(framework.bootstrap);
@layer framework.native {
/* most of our regular framework overrides can stay right here */
}
@layer framework.necessary {
/* add !necessary types in a decrease layer */
/* to override any !necessary framework types */
}
It nonetheless appears like a little bit of a hack, nevertheless it helps transfer us in the correct path — in the direction of a extra structured cascade. Hopefully it’s a brief repair.
Designing a CSS instrument or framework
For anybody sustaining a CSS library, cascade layers can assist with inner group, and even turn out to be a part of the developer API. By naming inner layers of a library, we are able to permit customers of our framework to hook into these layers when customizing or overriding our offered types.
For instance, Bootstrap may expose layers for his or her “reboot,” “grid,” and “utilities” — possible stacked in that order. Now a person can resolve in the event that they need to load these Bootstrap layers into totally different native layers:
@import url(bootstrap/reboot.css) layer(reset); /* reboot » reset.reboot */
@import url(bootstrap/grid.css) layer(structure); /* grid » structure.grid */
@import url(bootstrap/utils.css) layer(override); /* utils » override.utils */
Or the person would possibly load them right into a Bootstrap layer, with native layers interspersed:
@layer bs.reboot, bs.grid, bs.grid-overrides, bs.utils, bs.util-overrides;
@import url(‘bootstrap-all.css’) layer(bs);
It’s additionally doable to cover inner layering from customers, when desired, by grouping any non-public/inner layers inside an nameless (un-named) layer. Nameless layers will get added to the layer order the place they’re encountered, however is not going to be uncovered to customers re-arranging or appending types.
I simply need this one property to be extra !necessary
Counter to some expectations, layers don’t make it simple to shortly escalate a selected model in order that it overrides one other.
If the vast majority of our types are un-layered, then any new layer shall be de-prioritized in relation to the default. We may try this to particular person model blocks, however it could shortly turn out to be tough to trace.
Layers are supposed to be extra foundational, not style-by-style, however establishing constant patterns throughout a mission. Ideally, if we’ve set that up proper, we get the right consequence by transferring our model to the suitable (and pre-defined) layer.
If the vast majority of our types already fall into well-defined layers, we are able to at all times contemplate including a brand new highest-power layer on the prime of a given stack, or utilizing un-layered types to override the layers. We’d even contemplate having a debug layer on the prime of the stack, for doing exploratory work exterior of manufacturing.
However including new layers on-the-fly can defeat the organizational utility of this function, and must be used fastidiously. It’s finest to ask: Why ought to this model override the opposite?
If the reply has to do with one kind of favor at all times overriding one other kind, layers are in all probability the correct answer. That could be as a result of we’re overriding types that come from a spot we don’t management, or as a result of we’re writing a utility, and it ought to transfer into our utilities layer. If the reply has to do with extra focused types overriding much less focused types, we’d contemplate making the selectors replicate that specificity.
Or, on uncommon events, we’d even have types that basically are necessary — the function merely doesn’t work when you override this specific model. We’d say including show: none to the [hidden] attribute belongs in our lowest-priority reset, however ought to nonetheless be exhausting to override. In that case, !necessary actually is the correct instrument for the job:
@layer reset {
[hidden] { show: none !necessary; }
}
Scoping and name-spacing types? Nope!
Cascade layers are clearly an organizational instrument, and one which ‘captures’ the affect of selectors, particularly after they battle. So it may be tempting at first look to see them as an answer for managing scope or name-spacing.
A standard first-instinct is to create a layer for every element in a mission — hoping that may guarantee (for instance) that .post-title is just utilized inside a .publish.
However cascade conflicts should not the identical as naming conflicts, and layers aren’t significantly nicely designed for one of these scoped group. Cascade layers don’t constrain how selectors match or apply to the HTML, solely how they cascade collectively. So except we are able to make sure that element X at all times override element Y, particular person element layers gained’t assist a lot. As an alternative, we’ll have to control the proposed @scope spec that’s being developed.
It may be helpful to consider layers and component-scopes as a substitute as overlapping considerations:
Scopes describe what we’re styling, whereas layers describe why we’re styling. We are able to additionally consider layers as representing the place the model comes from, whereas scopes symbolize what the model will connect to.
Check your data: Which model wins?
For every state of affairs, assume this paragraph:
<p id=”intro”>Hey, World!</p>
Query 1
@layer ultra-high-priority {
#intro {
colour: pink;
}
}
p {
colour: inexperienced;
}
What colour is the paragraph?
Regardless of the layer having a reputation that sounds fairly necessary, un-layered types have the next precedence within the cascade. So the paragraph shall be inexperienced.
Query 2
@layer ren, stimpy;
@layer ren {
p { colour: pink !necessary; }
}
p { colour: inexperienced; }
@layer stimpy {
p { colour: blue !necessary; }
}
What colour is the paragraph?
Our regular layer order is established initially — ren on the backside, then stimpy, then (as at all times) un-layered types on the prime. However these types aren’t all regular, a few of them are necessary. Instantly, we are able to filter down to only the !necessary types, and ignore the unimportant inexperienced. Do not forget that ‘origins and significance’ are step one of the cascade, earlier than we even take layering under consideration.
That leaves us with two necessary types, each in layers. Since our necessary layers are reversed, ren strikes to the highest, and stimpy to the underside. The paragraph shall be pink.
Query 3
@layer Montagues, Capulets, Verona;
@layer Montagues.Romeo { #intro { colour: pink; } }
@layer Montagues.Benvolio { p { colour: orange; } }
@layer Capulets.Juliet { p { colour: yellow; } }
@layer Verona { * { colour: blue; } }
@layer Capulets.Tybalt { #intro { colour: inexperienced; } }
What colour is the paragraph?
All our types are in the identical origin and context, none are marked as necessary, and none of them are inline types. We do have a broad vary of selectors right here, from a extremely particular ID #intro to a zero specificity common * selector. However layers are resolved earlier than we take specificity under consideration, so we are able to ignore the selectors for now.
The first layer order is established up entrance, after which sub-layers are added internally. However sub-layers are sorted together with their father or mother layer — which means all of the Montagues can have lowest precedence, adopted by all of the Capulets, after which Verona has remaining say within the layer order. So we are able to instantly filter down to only the Verona types, which take priority. Regardless that the * selector has zero specificity, it’s going to win.
Watch out about placing common selectors in highly effective layers!
Debugging layer conflicts in browser developer instruments
Chrome, Safari, Firefox, and Edge browsers all have developer instruments that let you examine the types being utilized to a given factor on the web page. The types panel of this factor inspector will present utilized selectors, sorted by their cascade precedence (highest precedence on the prime), after which inherited types under. Types that aren’t being utilized for any cause will typically be grayed out, and even crossed out — typically with extra details about why the model just isn’t utilized. That is the primary place to look when debugging any side of the cascade, together with layer conflicts.
Safari Know-how Preview and Firefox Nightly already present (and type) cascade layers on this panel. This tooling is anticipated to function out within the secure variations concurrently cascade layers. The layer of every selector is listed instantly above the selector itself:
Chrome/Edge are engaged on related instruments and count on to have them obtainable in Canary (nightly) releases by the point cascade layers land within the secure launch. We’ll make updates right here as these instruments change and enhance.
Browser assist and fallbacks
Cascade layers are (or will quickly be) obtainable by default in all of the three main browser engines:
Chrome/Edge 99+Firefox 97+Safari (at the moment within the Know-how Preview)
Since layers are supposed as foundational constructing blocks of a whole CSS structure, it’s tough to think about constructing handbook fallbacks in the identical approach you would possibly for different CSS options. The fallbacks would possible contain duplicating massive sections of code, with totally different selectors to handle cascade layering — or offering a a lot easier fallback stylesheet.
Question function assist utilizing @helps
There’s a @helps function in CSS that may permit authors to check for assist of @layer and different at-rules:
@helps at-rule(@layer) {
/* code utilized for browsers with layer assist */
}
@helps not at-rule(@layer) {
/* fallback utilized for browsers with out layer assist */
}
Nevertheless, it’s additionally not clear when this question itself shall be supported in browsers.
Assigning layers in HTML with the <hyperlink> tag
There isn’t any official specification but for a syntax to layer whole stylesheets from the html <hyperlink> tag, however there’s a proposal being developed. That proposal features a new layer attribute which can be utilized to assign the types to a named or nameless layer:
<!– types imported into to the <layer-name> layer –>
<hyperlink rel=”stylesheet” href=”instance.css” layer=”<layer-name>”>
<!– types imported into to a brand new nameless layer –>
<hyperlink rel=”stylesheet” href=”instance.css” layer>
Nevertheless, previous browsers with out assist for the layer attribute will ignore it fully, and proceed to load the stylesheet with none layering. The outcomes might be fairly surprising. So the proposal additionally extends the prevailing media attribute, in order that it permits function assist queries in a assist() operate.
That might permit us to make layered hyperlinks conditional, primarily based on assist for layering:
<hyperlink rel=”stylesheet” layer=”bootstrap” media=”helps(at-rule(@layer))” href=”bootstrap.css”>
Potential polyfills and workarounds
The key browsers have all moved to an “evergreen” mannequin with updates pushed to customers on a reasonably quick launch cycle. Even Safari recurrently releases new options in “patch” updates between their extra rare-seeming main variations.
Meaning we are able to count on browser assist for these options to ramp up in a short time. For many people, it could be affordable to begin utilizing layers in only some months, with out a lot concern for previous browsers.
For others, it could take longer to really feel snug with the native browser assist. There are numerous different methods to handle the cascade, utilizing selectors, customized properties, and different instruments. It’s additionally theoretically doable to imitate (or polyfill) the essential conduct. There are individuals engaged on that polyfill, nevertheless it’s not clear when that shall be prepared both.
Extra sources
CSS Cascade Layers remains to be evolving however there’s already lots of sources, together with documentation, articles, movies, and demos that can assist you get much more accustomed to layers and the way they work.
Reference
Cascading & Inheritance Stage 5 SpecificationMDN Documentation
Articles
The Way forward for CSS: Cascade Layers (CSS @layer) by Bramus Van DammeGetting Began With CSS Cascade Layers by Stephanie Eckles, Smashing JournalCascade layers are coming to your browser by Una Kravets, Chrome Builders
Movies
How does CSS !necessary really work? by Una KravetsAn summary of the brand new @layer and layer() CSS primitives by Una KravetsCSS Revert & Revert-Layer Key phrases by Una Kravets
Demos
A Full Information to CSS Cascade Layers initially revealed on CSS-Methods. You need to get the publication.
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