You can set if it’s reduced motion in Chrome easily by opening dev tools, pressing the three dots in the top right, then going on more tools > rendering and finding the “Emulate css media feature prefers reduced motion”

Reduced Motion: false

Code for demo above:

<script>
    import { reducedMotion, createTransition } from 'svelte-reduced-motion';
    import { fly } from 'svelte/transition';

    let showing;

    const accessibleTransition = createTransition(fly);
</script>

<p>
    Reduced Motion: {$reducedMotion}
</p>

<button on:click={() => (showing = !showing)}> Toggle Animation </button>

{#if showing}
    <div
        transition:accessibleTransition={{ y: -20 }}
        style="padding: 16px 0px;">
        Hello World
    </div>
{/if}

You can also simplify this by using the prebuilt transitions:

<script>
    import { fly } from 'svelte-reduced-motion/transition';

    let showing;
</script>

<button on:click={() => (showing = !showing)}> Toggle Animation </button>

{#if showing}
    <div
        transition:fly={{ y: -20 }}
        style="padding: 16px 0px;">
        Hello World
    </div>
{/if}