Day 84: the @property at-rule
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It’s time to get me up to speed with modern CSS. There’s so much new in CSS that I know too little about. To change that I’ve started #100DaysOfMoreOrLessModernCSS. Why more or less modern CSS? Because some topics will be about cutting-edge features, while other stuff has been around for quite a while already, but I just have little to no experience with it.
The @property
rule allows you to register custom properties.
You can already define custom properties, but the difference between defining and registering is that the at-rule allows you to specify the type and other attributes.
@property --hue {
/* The type. */
syntax: '<angle>';
/* Is it an inhertiable property? */
inherits: false;
/* The initial value. */
initial-value: 0deg;
}
syntax
The syntax
descriptor specifies the syntax (or type) of the property. You can find a list of supported syntax component names in the spec.
@property --milliseconds {
syntax: '<integer>';
inherits: false;
}
inherits
The inherits
descriptor specifies whether the property inherits from its parent or not.
@property --color-primary {
syntax: '<color>';
inherits: true;
}
initial-value
The initial-value
descriptor specifies the initial value of the custom property.
@property --color-primary {
syntax: '<color>';
inherits: true;
initial-value: rgb(0 0 0);
}
An example
Let's say we have a <button>
and we want to transition the background color on :hover
and :focus-visible
.
button {
--h: 176;
--s: 74%;
--l: 60%;
--bg: hsl(var(--h) var(--s) var(--l));
background-color: var(--bg);
transition: background-color 1s;
}
button:is(:hover, :focus-visible) {
--h: 20;
}
That works well, we get a nice transition from the first color to the second color, but if we want to animate just the hue to get a more interesting effect, we have bad luck, because the value of --h
is a string, which you can't animate.
button {
--h: 176;
--s: 74%;
--l: 60%;
--bg: hsl(var(--h) var(--s) var(--l));
background-color: var(--bg);
transition: --h 1s;
}
With @property
we can turn the string into number and animate it.
@property --h {
initial-value: 0;
inherits: true;
syntax: '<number>';
}
button {
--h: 176;
--s: 74%;
--l: 60%;
--bg: hsl(var(--h) var(--s) var(--l));
background-color: var(--bg);
transition: --h 1.6s;
}
Further reading
Do you want to learn even more awesome CSS?
I'm running a workshop with my friends at Smashing Magazine in which I introduce you to the most useful modern features in CSS and show how you can implement them today in your code base to improve scalability, maintainability, and productivity.
Learn more about the workshop!
Overview: 100 Days Of More Or Less Modern CSS