document.addEventListener("DOMContentLoaded", function() { const target = document.querySelectorAll('.fade-in-target'); const observer = new IntersectionObserver((entries) => { entries.forEach(entry => { if (entry.isIntersecting) { entry.target.classList.add('is-active'); // ここでCSSアニメーションが発火 } }); }); target.forEach(el => observer.observe(el)); });