<div id="modal-overlay" class="modal-overlay">
  <div class="modal">
    <div class="modal-header">
      <h3 id="modal-title">标题</h3>
      <span class="modal-close" onclick="closeModal()">&times;</span>
    </div>
    <div class="modal-body" id="modal-content">
      这里是模态框的内容。
    </div>
    <div class="modal-footer">
      <button onclick="closeModal()">关闭</button>
    </div>
  </div>
</div>

.modal-overlay {
  display: none;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0, 0, 0, 0.5);
  justify-content: center;
  align-items: center;
  z-index: 1000;
}

.modal {
  background: white;
  border-radius: 8px;
  width: 400px;
  max-width: 90%;
  box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
  overflow: hidden;
}

.modal-header {
  padding: 16px;
  border-bottom: 1px solid #eee;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.modal-header h3 {
  margin: 0;
  font-size: 1.2em;
}

.modal-close {
  font-size: 24px;
  cursor: pointer;
  color: #aaa;
}

.modal-close:hover {
  color: #000;
}

.modal-body {
  padding: 16px;
  line-height: 1.5;
}

.modal-footer {
  padding: 16px;
  border-top: 1px solid #eee;
  text-align: right;
}

.modal-footer button {
  padding: 8px 16px;
  border: none;
  background-color: #007bff;
  color: white;
  border-radius: 4px;
  cursor: pointer;
}

.modal-footer button:hover {
  background-color: #0056b3;
}

function openModal(title, content) {
  document.getElementById("modal-title").textContent = title;
  document.getElementById("modal-content").innerHTML = content;
  document.getElementById("modal-overlay").style.display = "flex";
}

function closeModal() {
  document.getElementById("modal-overlay").style.display = "none";
}

// 可选：点击遮罩层关闭模态框
document.getElementById("modal-overlay").addEventListener("click", function(e) {
  if (e.target === this) {
    closeModal();
  }
});

<button onclick='openModal("欢迎", "这是一个简单的模态框示例。")'>
  打开模态框
</button>