svelte

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title
Styles & Classes

style scoping
:global
style:
class:
--css props

Styling is a fundamental part of UI components. Svelte helps you style your components with ease, providing useful features out of the box.

Scoped by default

By default CSS inside a <style> block will be scoped to that component.

This works by adding a class to affected elements, which is based on a hash of the component styles (e.g. svelte-123xyz).

<style>
	p {
		/* this will only affect <p> elements in this component */
		color: burlywood;
	}
</style>

:global(...)

To apply styles to a single selector globally, use the :global(...) modifier:

<style>
	:global(body) {
		/* applies to <body> */
		margin: 0;
	}

	div :global(strong) {
		/* applies to all <strong> elements, in any component,
		   that are inside <div> elements belonging
		   to this component */
		color: goldenrod;
	}

	p:global(.big.red) {
		/* applies to all <p> elements belonging to this component
		   with `class="big red"`, even if it is applied
		   programmatically (for example by a library) */
	}
</style>

If you want to make @keyframes that are accessible globally, you need to prepend your keyframe names with -global-.

The -global- part will be removed when compiled, and the keyframe will then be referenced using just my-animation-name elsewhere in your code.

<style>
	@keyframes -global-my-animation-name {
		/* code goes here */
	}
</style>

:global

To apply styles to a group of selectors globally, create a :global {...} block:

<style>
	:global {
		/* applies to every <div> in your application */
		div { ... }

		/* applies to every <p> in your application */
		p { ... }
	}

	.a :global {
		/* applies to every `.b .c .d` element, in any component,
		   that is inside an `.a` element in this component */
		.b .c .d {...}
	}
</style>

[!NOTE] The second example above could also be written as an equivalent .a :global .b .c .d selector, where everything after the :global is unscoped, though the nested form is preferred.

Nested style tags

There should only be 1 top-level <style> tag per component.

However, it is possible to have a <style> tag nested inside other elements or logic blocks.

In that case, the <style> tag will be inserted as-is into the DOM; no scoping or processing will be done on the <style> tag.

<div>
	<style>
		/* this style tag will be inserted as-is */
		div {
			/* this will apply to all `<div>` elements in the DOM */
			color: red;
		}
	</style>
</div>

class:name

<!--- copy: false --->
class:name={value}

<!--- copy: false --->
class:name

A class: directive provides a shorter way of toggling a class on an element.

<!-- These are equivalent -->
<div class={isActive ? 'active' : ''}>...</div>
<div class:active={isActive}>...</div>

<!-- Shorthand, for when name and value match -->
<div class:active>...</div>

<!-- Multiple class toggles can be included -->
<div class:active class:inactive={!active} class:isAdmin>...</div>

style:property

<!--- copy: false --->
style:property={value}

<!--- copy: false --->
style:property="value"

<!--- copy: false --->
style:property

The style: directive provides a shorthand for setting multiple styles on an element.

<!-- These are equivalent -->
<div style:color="red">...</div>
<div style="color: red;">...</div>

<!-- Variables can be used -->
<div style:color={myColor}>...</div>

<!-- Shorthand, for when property and variable name match -->
<div style:color>...</div>

<!-- Multiple styles can be included -->
<div style:color style:width="12rem" style:background-color={darkMode ? 'black' : 'white'}>...</div>

<!-- Styles can be marked as important -->
<div style:color|important="red">...</div>

When style: directives are combined with style attributes, the directives will take precedence:

<div style="color: blue;" style:color="red">This will be red</div>

--style-props

<!--- copy: false --->
--style-props="anycssvalue"

You can also pass styles as props to components for the purposes of theming, using CSS custom properties.

Svelte's implementation is essentially syntactic sugar for adding a wrapper element. This example:

<Slider bind:value min={0} --rail-color="black" --track-color="rgb(0, 0, 255)" />

Desugars to this:

<div style="display: contents; --rail-color: black; --track-color: rgb(0, 0, 255)">
	<Slider bind:value min={0} max={100} />
</div>

For SVG namespace, the example above desugars into using <g> instead:

<g style="--rail-color: black; --track-color: rgb(0, 0, 255)">
	<Slider bind:value min={0} max={100} />
</g>

[!NOTE] Since this is an extra <div> (or <g>), beware that your CSS structure might accidentally target this. Be mindful of this added wrapper element when using this feature.

Svelte's CSS Variables support allows for easily themeable components:

<style>
	.potato-slider-rail {
		background-color: var(--rail-color, var(--theme-color, 'purple'));
	}
</style>

So you can set a high-level theme color:

/* global.css */
html {
	--theme-color: black;
}

Or override it at the consumer level:

<Slider --rail-color="goldenrod" />

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