JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://rawgit.com/ractivejs/ractive-transitions-slide/master/dist/ractive-transitions-slide.js"></script>
<main></main>

<script id='template' type='text/ractive'>
    <div intro='slide'>does not slide in - correct</div>
    <Widget>does not slide in - correct</Widget>
    
    {{#if later}}
        <Widget>does slide in - correct</Widget>
    {{/if}}
</script>

<script id='widget' type='text/ractive'>
    <div intro='slide'>{{yield}}</div>
</script>

CSS

body {
    font-family: 'Helvetica Neue', arial, sans-serif;
    font-weight: 200;
    color: #353535;
}

h1, h2, h3, h4, h5, h6 {
    font-weight: 200;
}

div {
    background-color: #ddd;
    padding: 1em;
    margin: 0 0 1em 0;
}

JavaScript

Ractive.components.Widget = Ractive.extend({ 
    template: '#widget' ,
    onconfig: function(){
        this.noIntro = !this.parent.fragment;
    }
});

var ractive = new Ractive({
    el: 'main',
    template: '#template',
    noIntro: true,
});

setTimeout( function () {
    ractive.set( 'later', true );
}, 1000 );