def METHOD_NAME(num_elements=1000):
    """Randomly generate an invalid HTML document."""
    tag_names = ['p', 'div', 'span', 'i', 'b', 'script', 'table']
    elements = []
    for i in range(num_elements):
        choice = random.randint(0,3)
        if choice == 0:
            # New tag.
            tag_name = random.choice(tag_names)
            elements.append("<%s>" % tag_name)
        elif choice == 1:
            elements.append(rsentence(random.randint(1,4)))
        elif choice == 2:
            # Close a tag.
            tag_name = random.choice(tag_names)
            elements.append("</%s>" % tag_name)
    return "<html>" + "\n".join(elements) + "</html>"