JavaScript
API Fluent

class Item {
    constructor(text, type) {   
        this.text = text;
        this.emphasis = false;
        this.type = type;
    }

    toHtml() {
        return `<${this.type}>${this.emphasis ? '<em>' : ''}${this.text}${this.emphasis ? '</em>' : ''}</${this.type}>`;
    }
}

class Section {
    constructor(header, paragraphs) {
        this.header = header;
        this.paragraphs = paragraphs;
    }
    
    toHtml() {
        return `<section><h2>${this.header}</h2>${this.paragraphs.map(p => p.toHtml()).join('')}</section>`;
    }
}

class List {
    constructor(text, items) {
        this.text = text;
        this.items = items;
    }
    
    toHtml() {
        return `<ol><h2>${this.text}</h2>${this.items.map(i => i.toHtml()).join('')}</ol>`;
    }
}

class Article {
    constructor(topic) {
        this.topic = topic;
        this.sections = [];
        this.lists = [];
    }

    section(text) {
        const section = new Section(text, []);
        this.sections.push(section);
        this.lastSection = section;
        return this;
    }
    
    list(text) {
        const list = new List(text, []);
        this.lists.push(list);
        this.lastList = list;
        return this;
    }

    addParagraph(text) {
        const paragraph = new Item(text, 'p');
        this.lastSection.paragraphs.push(paragraph);
        this.lastItem = paragraph;
        return this;
    }

    addListItem(text) {
        const listItem = new Item(text, 'li');
        this.lastList.items.push(listItem);
        this.lastItem = listItem;
        return this;
    }

    withEmphasis() {
        this.lastItem.emphasis = true;
        return this;
    }
    
    toHtml() {
        return `<article><h1>${this.topic}</h1>${this.sections.map(s => s.toHtml()).join('')}${this.lists.map(l => l.toHtml()).join('')}</article>`;
    }
}

Article.withTopic = topic => new Article(topic);

const articles = [
    Article.withTopic('Artificial Intelligence - Overview')
      .section('What is Artificial Intelligence?')
        .addParagraph('Something something')
        .addParagraph('Lorem ipsum')
          .withEmphasis()
      .section('Philosophy of AI')
          .addParagraph('Something about AI philosophy')
          .addParagraph('Conclusion'),
      
    Article.withTopic('JavaScript')
      .list('JavaScript is one of the 3 languages all web developers must learn:')
          .addListItem('HTML to define the content of web pages')
          .addListItem('CSS to specify the layout of web pages')
          .addListItem(' JavaScript to program the behavior of web pages')
];

document.getElementById('content').innerHTML = articles.map(a => a.toHtml()).join('\n');