JSFiddle - React, Tailwind, and code Playground

HTML

<section id="madplan" class="section-wrapper">

        <section class="check-list">
            <h1>Mandag</h1>
            <list-component 
              v-on:addedtask='acknowledgeAddedTask'
              v-on:removedtask='acknowledgeRemovedTask'
               ></list-component>
            <h1>Tirsdag</h1>
            <list-component 
              v-on:addedtask='acknowledgeAddedTask'
              v-on:removedtask='acknowledgeRemovedTask'
               ></list-component>

        </section>
        <ul id="list">
            <h2>list</h2>
            <li v-for="task in subTaskList" v-bind:key="task.text" class="list-item">{{ task.text }}</li>
        </ul>

    </section>

CSS

* {
  margin: 0;
  padding: 0;
}

JavaScript

Vue.component('list-component', {
            data: function() {
                return {
                    newTask: "",
                    taskList: [],
                    newSubTask: "",
                    subTaskList: [],
                };
            },
            template:
                '<div>' +

                '<section class="prefetch panel">' +
                '<input  v-if="showInput" class="input typeahead" type="text" placeholder="Tilføj ret til madplanen" v-model="newTask" v-on:keyup.enter="addTask">' +
                '</section>' +

                '<details v-for="task in taskList" v-bind:key="task.text" class="sub-list-item">' +
                '<summary>{{ task.text }}<button v-on:click="removeSubTask(task)">X</button>' + '</summary>' +
                '<input class="subInput" type="text" placeholder="Tilføj til indkøbsseddel" v-model="newSubTask" v-on:keyup.enter="addSubTask">' +
                '</details>' +

                '</div>',

            computed: {
                showInput: function() {
                    return !this.taskList.length
                },
            },

            methods: {
                //addTasks
                //
                addTask: function() {
                    var task = this.newTask.trim();
                    if (task) {
                        this.taskList.push({
                            text: task
                        });
                        this.newTask = "";
                        this.$emit('addedtask', task)
                    }
                },

                addSubTask: function() {
                    var task = this.newSubTask.trim();
                    if (task) {
                        this.subTaskList.push({
                            text: task
                        });
                        this.newSubTask = "";
                    }
                },

                //removeTasks
                //
                removeSubTask: function(task)...