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You can add clientWidth, clientHeight, offsetWidth and offsetHeight bindings to any element, and Svelte will update the bound values using a ResizeObserver:

App
<div bind:clientWidth={w} bind:clientHeight={h}>
	<span style="font-size: {size}px" contenteditable>{text}</span>
	<span class="size">{w} x {h}px</span>
</div>

These bindings are readonly — changing the values of w and h won’t have any effect on the element.

display: inline elements do not have a width or height (except for elements with ‘intrinsic’ dimensions, like <img> and <canvas>), and cannot be observed with a ResizeObserver. You will need to change the display style of these elements to something else, such as inline-block.

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<script>
	let w = $state();
	let h = $state();
	let size = $state(42);
</script>
 
<label>
	<input type="range" bind:value={size} min="10" max="100" />
	font size ({size}px)
</label>
 
<div>
	<span style="font-size: {size}px" contenteditable>
		edit this text
	</span>
 
	<span class="size">{w} x {h}px</span>
</div>
 
<style>
	div {
		position: relative;
		display: inline-block;
		padding: 0.5rem;
		background: hsla(15, 100%, 50%, 0.1);
		border: 1px solid hsl(15, 100%, 50%);
	}
 
	.size {
		position: absolute;
		right: -1px;
		bottom: -1.4em;
		line-height: 1;
		background: hsl(15, 100%, 50%);
		color: white;
		padding: 0.2em 0.5em;
		white-space: pre;
	}
</style>