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February 27, 2024

Smashing Workshop

Variables & Value Resolution

controlling the matrix

workshops.oddbird.net/smashing24/variables/

Thanks to… The OddBird Team

(especially Stacy & David)

The team at OddBird helped put this together
Building the site & tooling
Stacy Kvernmo & David Herron built demos
And have been around helping out
There’s nothing like being on a team
(say hi?)

Quick poll (in chat)… Who’s Using Custom Properties?

✅ all the time
🤔 on occasion
❌ not at all

Variables Store Values

Variables are common in programming
Capture, store, and access values

Design Tokens

brand colors, sizes, fonts, etc…

In CSS that has often meant
Global ‘design tokens’
brand colors, sizes, fonts, etc

Design Tokens

button-background, primary-grid…

Don’t have to be global
Don’t have to be simple

Re-use to Avoid Repetition

Broadly any CSS value we want to re-use

Provide Meaningful Names

`hsl(331deg 42% 30%)`

What is this color?
A mix of darkRed and mediumBlue?
With variables we can give it
A meaningful name, based on…

‘`deep-raspberry`’

What it looks like
(deep raspberry)

Why we chose it
(the best color)

What it signifies
(action, maybe)

‘`link-text`’
‘`button-background`’

Where we’ll use it
(link text and button backgrounds?)

scss…

--deep-raspberry: hsl(322, 92%, 24%);
--best-color: var(--deep-raspberry);
--action: var(--best-color);
--link-text: var(--action);
--button-background: var(--action);

Or all of the above?
We can use one variable to name the next
Creating different layers of meaning

Like audio patch bays
Connecting an input to an output
With a physical cable
Like audio in cables…

Don’t Do Anything

Until we apply them

You don’t hear any sound
Until you plug it into a speaker of some kind
An amplifier,
Something that turns signal into noise

scss…

button {
  background-color: var(--button-background);
}

In CSS
Normal properties are the amplifier
Variables carry silent tokens
Until we plug them in
And the normal property makes it a style

That’s when things get interesting

Widely supported since 2016…

Two names
Useful as CSS variables
But they are also… properties
Custom properties, that we define

Properties Of Elements

(in the cascade)

Like any other property in CSS
Cascade, inherit, and resolve
Expose the inner workings of value resolution!
(Force us to learn it)

css…

.warning {
  --state-color: maroon;
}

`--``name:` `value``;`

Using a normal declaration syntax,
(property and value)
But we come up with the property name
Requirement: start with a double-dash

Called `--dashed-idents`

For mixing custom & built-in identifiers

in CSS, called ‘dashed idents’
Not only used for custom props
But anywhere a ‘custom ident’
(something we name)
Shares space with other CSS keywords

css…

built-in-property: value;
--custom-property: value;

In rule blocks, mixing declarations
Some built-in, some custom properties
Need to tell the difference
Avoid conflicts in the future

`-``webkit``-property`
`-``moz``-property`
`-``ms``-property` `--property`

The syntax comes from browser prefixes
Which have (mostly) been phased out
Remove the browser name
Now it’s ours

css…

.warning {
  --state-color: maroon;
}

Apply to any elements
But they won’t do anything…
(except cascade and inherit)

css…

.alert {
  border: medium solid var(--state-color);
}

Until we access them
Apply them to a normal property…

`var(` `--property` `)`

Using the var function
Var, parenthesis, dashed-ident
The name of the custom property to access

css…

.ಠ_ಠ {
  --（╯°□°）╯: ︵┻━┻;
}

Beyond the double-dash,
Extremely permissive syntax
This table-flipping emoji is valid CSS
Just not very useful

Unless maybe…
(We’ll dig into that one next week)

Scott Kellum takes advantage of this
In CJSS
Allowing you to write JS and HTML
Directly in CSS!
(I promise, some useful demos)

No… Non-important `!`

A few limitations…

No… Un-Matched Closing Bracket ), ], }

Unlike other CSS properties, custom property names are not ASCII case-insensitive.

— CSS Custom Properties Level 1

Properties Of Elements

Otherwise normal properties
Of elements
(don’t exist or have a value outside)

Follow Normal Cascade

Follow normal cascade
But highlight some strange edge-cases…

But Not Filtering… For each HTML Element,
Find All Relevant Declarations

In the filtering step, before cascading…

Must be Syntactically Valid

(otherwise discarded at parse-time)

Declarations must be syntactically valid
But what’s that mean?

css…

.ಠ_ಠ {
  --（╯°□°）╯: ︵┻━┻;
}

According to CSS syntax rules
Valid to define

css…

.ಠ_ಠ {
  --（╯°□°）╯: ︵┻━┻;
  color: var(--（╯°□°）╯);
}

Valid to access on (say) color prop

css…

.ಠ_ಠ {
  color: ︵┻━┻;
}

Only becomes invalid
After we replace the variable
Color property requires color syntax

`var()` output… Not Validated
Until Applied

Parse-time validation early, before cascade
We don’t know cascaded value

Variable substitution After Cascade is Complete

Var substitution has to wait
Go by syntax alone

css…

.ಠ_ಠ {
  --（╯°□°）╯: ︵┻━┻;
  color: var(--（╯°□°）╯);
}

Valid, but will fail later
Will fail after the cascade
Which is a problem…

css…

button {
  --fancy-color: oklch(0.8 0.1 0);

  color: pink;
  color: var(--fancy-color);
}

If we have a fancy color in the var
And try to provide a fallback
By repeating the property…

css…

button {
  --fancy-color: oklch(0.8 0.1 0);

  /* color: pink; DISCARDED by cascade */
  color: var(--fancy-color);
}

Fallback will be removed by the cascade
Before we know if the fancy color is supported

css…

button {
  --fancy-color: oklch(0.8 0.1 0);

  /* color: pink; DISCARDED by cascade */
  /* color: var(--fancy-color); INVALID */
}

In a browser without OKLCH
The variable will fail when subbed
We end up without any color!

Properties using `var()` become… Invalid At Computed Value Time

It doesn’t have a value
Solution:
Use @supports when setting variables

Cascading Removes Extra Values

result: 0-1 cascaded values

The cascade filtered us down
Ensure we don’t have more than one value
But we might still have no value!

We need… Single Value Per Element

Defaulting Inserts Missing Values

result: one specified value

Defaulting process Depends on the Property

Some Properties Inherit

value from direct parent

Values may be specified much higher up
Still only have to look at direct parent
Styles resolve in document order, outside-in
Each level, until a cascaded value
Because the cascade happened first

Start with all these nested boxes
Assume they are un-styled boxed

Cascade goes first
Explicit selectors & declarations
Styles attached to individual elements
Not everything gets style…

So after the cascade is complete,
And only when values are missing,
Then we apply inheritance
Each (un-styled) box looking at it’s parent
Cascade has priority (by going first)

Cascaded values Based on Selector Mapping

Inherited values Based on DOM Context

Custom properties Inherit (By Default)

css…

main { --action: maroon; }
aside { --action: teal; }
footer { --action: rebeccaPurple; }

button { background: var(--action); }

We can Reduce Nesting

css…

button { background: blue; }
main button { background: maroon; }
aside button { background: teal; }
main aside button { background: hotPink; }

Inheritance Rewards Proximity

Allow us to Control Inheritance

We can Capture a Value

anywhere in the DOM

We can Access a Value

from any ancestor

[demo]:

cascaded declarations first
(our element is the subject)
(sorted by cascade priority)
inherited declarations
(our element is a descendant)
(sorted by inheritance proximity)
together: all the relevant declarations
and which one applies!
computed… still missing values for most props

Most properties Don’t Inherit

If I added a span in the middle of this paragraph, what styles would I expect to apply automatically inside the span?

— Internal Monologue

Generally… Un-styled Inline Boxes `(spans)`
Should Not Impact Layout

Generally… Layouts Do Not Inherit

(including anything box-related)

Generally… Typography Inherits

(including text colors)

No inheritance? Use Initial Value

Spec here is brief
We’ll read it!
(copied here)

Each property has an initial value, defined in the property’s definition table.

— Cascade & Inheritance, § 7.1. Initial Values

[Demo]:

pick a property (display?)
Look it up on MDN
Check ‘formal definition’
(shows inherited, and initial)
Link to spec
(same)

If the property is not an inherited property, and the cascade does not result in a value, then the specified value of the property is its initial value.

— Cascade & Inheritance, § 7.1. Initial Values

We’ve Done It!* 🥳 Exactly One Specified Value

Either from…

The Cascade (declarations)
Inheritance (parent)
Initial Value (spec)

We’ve Done It!* (* Almost)

explicit defaulting… “Global Keywords”

available on every CSS property

Global Keywords…

initial
inherit
unset (inherit or initial)
revert (previous cascade origin)
revert-layer (previous cascade layer)

`initial` & `inherit`

choose your default, explicitly

Only use these, if you know what you want

`unset`

whichever default is appropriate

Pretend no one set any value
Prefer this over initial/inherit

css…

body { margin: unset; }

This is a great value for CSS resets

`revert` & `revert-layer`

defer to previous origin or layer

Often what you really want
Pretend I didn’t set anything

initial value is not browser default
be wary of globally inheriting box properties
usually use unset or one of the revert values

Global keywords… Resolve on Custom Properties

Global keywords on all properties
Including custom properties

css…

html {
  --color: teal;
  --color: initial;
  background: black;
  background: var(--color, red);
}

No way to pass keywords along as values

Initial Value Of A Custom Property?

css…

html {
  font-size: var(--i-never-specified-a-value);
}

Guaranteed Invalid Value

like Undefined in JS

We call it ‘the guaranteed invalid value’
No syntax for it
Only used for custom properties
And this will surprise you…
it’s guaranteed not to be valid

Properties using `var()` become… Invalid At Computed Value Time

result: the property is un-set

If it ends up on a property, IACVT
Since it’s guaranteed, we know earlier in the process
Not early enough for the cascade
But early enough for substitution…

css…

var(--undefined, fallback)

Allows us to provide fallbacks

css…

var(--my-color, teal)

css…

/* fallback: 'Georgia, Palatino, serif' */
var(--my-font, Georgia, Palatino, serif)

css…

var(--btn-color, var(--action, teal))

Warning Not for Property Invalid

Before validation on the property
So it only works with GIV

Warning Not for Legacy Support

Still can’t use it for legacy support fallbacks

css…

html { --my-color: red; }

@supports (color: oklch(0% 0% 0)) {
  html { --my-color: oklch(65% 0.2625 350.47); }
}

css…

html {
  --guaranteed-invalid: initial;
}

(ok, now) We’ve Done It! 🥳 Exactly One Specified Value

The Cascade (declarations)
Inheritance (parent)
Initial Value (spec)
Explicit Default (keyword)

Next step… Computed Value

making relative values absolute

css…

p {
  color: currentColor; /* rgb(0, 0, 0) ? */
  font-size: 1.2em; /* 19.2px ? */
  border-color: hotPink; /* rgb(255, 105, 180) */
  width: 80%; /* 80% (requires layout) */
  height: auto; /* auto (requires layout) */
}

Computed Values Inherit

`currentColor` Inherits as Keyword

It should re-calculate when the color changes!

Custom Properties… Mostly As Specified

css…

p {
  --my-size: 1.5lh; /* 1.5em */
  --my-color: hotPink; /* hotPink */
}

Value hasn’t been parsed/validated
Not clear yet what type this is
Can’t make it absolute without a type

Custom Properties… `var()` Substitution

css…

p {
  --my-size: 1.5lh; /* 1.5em */
  --alias: var(--my-size); /* 1.5em */
  --calc: calc(var(--alias) * 2); /* calc(1.5lh * 2) */
}

Properties… Validated & Compute `var()`

css…

p {
  --calc: calc(var(--alias) * 2); /* calc(1.5lh * 2) */
  color: var(--calc); /* unset */
  min-height: var(--calc); /* 3lh */
}

demo:

see ‘computed’ panel
custom properties aren’t resolving?
only one! let’s look into that…

Custom Property Definition?

We’ve talked about defaults
We can register custom properties
And define more about them

css…

@property --brand-color {
  syntax: "<color>";
  inherits: true;
  initial-value: hotPink;
}

Can I use… @property

All three Descriptors Required

syntax, inherits, and (usually) initial

`inherits` `true` or `false`

css…

/* replace 'initial' with any default value */
* { --this-property: initial; }

Doesn’t actually stop inheritance
Just ensures we never get to it
Always a declared value
Reset on every element

`syntax`… Supported Names

Or "*" for ‘universal syntax’

Does Not Fix Invalid At Computed Value Time

Definitions are still not
… available at parse time
… stable enough to rely on

Does Change Computed Values

(depending on syntax)

demo:

remove and re-add registration
inspect computed values
change to universal syntax (doesn’t compute)
change to color syntax (invalid)

Also… Allows Transitions & Animations

Currently… No Changing Types

Numbers, Lengths, & Strings

css…

.number-to-length {
  --number: 1;
  --length: calc( var(--number) * 1em);
  /* not `var(--number)em` */
}

css…

.number-to-length {
  --length: 1em;
  --number: calc( var(--length) / 1em );
}

`initial-value`… Must be Absolute

(when the syntax is not *)

Initial ~= Spec
Root/**`*`** ~= Browser

initial values equivalent to spec initial
Not right for every situation,
But a good fallback
Declarations on root/* better for dynamic tokens

Final Steps Used & Actual Values

All properties that apply
But some props might not apply
(e.g. flex: 1 on a non-flex item)
Usually the same as ‘computed’

css…

html {
  color-scheme: dark light;
  /* ~ used: dark; */
}

css…

div {
  width: 80%;
  /* ~ used: 1049.6px; */
  /* ~ actual: 1050px; */
}

🥳 Values Resolved

That’s it,
the entire ‘value resolution’ process
Any questions on that,
before we get into
more variable use-cases?

Custom properties can Store Global Tokens

css…

:root { --btn-color: maroon; }

These tokens will inherit everywhere

Inheritance will prioritize proximity
Change value for nested contexts

css…

:root { --btn-color: maroon; }
/* html { --btn-color: maroon; } */

Either root or html is fine
In any html document, select the same thing
Maybe you find it more clear?
Mostly just increases specificity
(Element type to Pseudo-Class)

Any Document Root

Like <svg> or <xml> or <html>

It’s true!
But is it actually what you want?

demo:

just an svg on it’s own
root styles apply to the svg element

demo:

same svg in a page
(styles commented out)
html styles targeting the root & svg…
what happens when we uncomment?

Variables & Value Resolution

Thanks to… The OddBird Team

Quick poll (in chat)… Who’s Using Custom Properties?

Variables Store Values

Design Tokens

Design Tokens

Re-use to Avoid Repetition

Provide Meaningful Names

hsl(331deg 42% 30%)

‘deep-raspberry’

‘best-color’

‘action’

‘link-text’ ‘button-background’

Don’t Do Anything

Properties Of Elements

--name: value;

Called --dashed-idents

-webkit-property -moz-property -ms-property --property

var( --property )

No… Non-important `!`

No… Un-Matched Closing Bracket ), ], }

Properties Of Elements

Follow Normal Cascade

But Not Filtering… For each HTML Element, Find All Relevant Declarations

Must be Syntactically Valid

var() output… Not Validated Until Applied

Variable substitution After Cascade is Complete

Properties using var() become… Invalid At Computed Value Time

Cascading Removes Extra Values

We need… Single Value Per Element

Defaulting Inserts Missing Values

Defaulting process Depends on the Property

Some Properties Inherit

Cascaded values Based on Selector Mapping

Inherited values Based on DOM Context

Custom properties Inherit (By Default)

We can Reduce Nesting

Inheritance Rewards Proximity

Allow us to Control Inheritance

We can Capture a Value

We can Access a Value

Most properties Don’t Inherit

Generally… Un-styled Inline Boxes (spans) Should Not Impact Layout

Generally… Layouts Do Not Inherit

Generally… Typography Inherits

No inheritance? Use Initial Value

We’ve Done It!* 🥳 Exactly One Specified Value

Either from…

We’ve Done It!* (* Almost)

explicit defaulting… “Global Keywords”

Global Keywords…

initial & inherit

unset

revert & revert-layer

Global keywords… Resolve on Custom Properties

Initial Value Of A Custom Property?

Guaranteed Invalid Value

Properties using var() become… Invalid At Computed Value Time

Warning Not for Property Invalid

Warning Not for Legacy Support

(ok, now) We’ve Done It! 🥳 Exactly One Specified Value

Next step… Computed Value

Computed Values Inherit

currentColor Inherits as Keyword

Custom Properties… Mostly As Specified

Custom Properties… var() Substitution

Properties… Validated & Compute var()

Custom Property Definition?

Can I use… @property

All three Descriptors Required

inherits true or false

syntax… Supported Names

Does Not Fix Invalid At Computed Value Time

Does Change Computed Values

Also… Allows Transitions & Animations

Currently… No Changing Types

Numbers, Lengths, & Strings

initial-value… Must be Absolute

Initial ~= Spec Root/* ~= Browser

Final Steps Used & Actual Values

`hsl(331deg 42% 30%)`

‘`deep-raspberry`’

‘`best-color`’

‘`action`’

‘`link-text`’
‘`button-background`’

`--``name:` `value``;`

Called `--dashed-idents`

`-``webkit``-property`
`-``moz``-property`
`-``ms``-property` `--property`

`var(` `--property` `)`

But Not Filtering… For each HTML Element,
Find All Relevant Declarations

`var()` output… Not Validated
Until Applied

Properties using `var()` become… Invalid At Computed Value Time

Generally… Un-styled Inline Boxes `(spans)`
Should Not Impact Layout

`initial` & `inherit`

`unset`

`revert` & `revert-layer`

Properties using `var()` become… Invalid At Computed Value Time

`currentColor` Inherits as Keyword

Custom Properties… `var()` Substitution

Properties… Validated & Compute `var()`

`inherits` `true` or `false`

`syntax`… Supported Names

`initial-value`… Must be Absolute

Initial ~= Spec
Root/**`*`** ~= Browser