// 获取<p id="p-id">...</p>
var p = document.getElementById('p-id');
// 设置文本为abc:
p.innerHTML = 'ABC'; // <p id="p-id">ABC</p>
// 设置HTML:
p.innerHTML = 'ABC <span style="color:red">RED</span> XYZ';
// <p>...</p>的内部结构已修改

// 获取<p id="p-id">...</p>
var p = document.getElementById('p-id');
// 设置文本:
p.innerText = '<script>alert("Hi")</script>';
// HTML被自动编码，无法设置一个<script>节点:
// <p id="p-id"><script>alert("Hi")</script></p>

// 获取<p id="p-id">...</p>
var p = document.getElementById('p-id');
// 设置CSS:
p.style.color = '#ff0000';
p.style.fontSize = '20px';
p.style.paddingTop = '2em';

<!-- HTML结构 -->
<p id="js">JavaScript</p>
<div id="list">
  <p id="java">Java</p>
  <p id="python">Python</p>
  <p id="scheme">Scheme</p>
</div>

var
  js = document.getElementById('js'),
  list = document.getElementById('list');
list.appendChild(js);

<!-- HTML结构 -->
<div id="list">
  <p id="java">Java</p>
  <p id="python">Python</p>
  <p id="scheme">Scheme</p>
  <p id="js">JavaScript</p>
</div>

var
  list = document.getElementById('list'),
  haskell = document.createElement('p');
haskell.id = 'haskell';
haskell.innerText = 'Haskell';
list.appendChild(haskell);

<!-- HTML结构 -->
<div id="list">
  <p id="java">Java</p>
  <p id="python">Python</p>
  <p id="scheme">Scheme</p>
  <p id="haskell">Haskell</p>
</div>

var d = document.createElement('style');
d.setAttribute('type', 'text/css');
d.innerHTML = 'p { color: red }';
document.getElementsByTagName('head')[0].appendChild(d);

<!-- HTML结构 -->
<div id="list">
  <p id="java">Java</p>
  <p id="python">Python</p>
  <p id="scheme">Scheme</p>
</div>

var
  list = document.getElementById('list'),
  ref = document.getElementById('python'),
  haskell = document.createElement('p');
haskell.id = 'haskell';
haskell.innerText = 'Haskell';
list.insertBefore(haskell, ref);

<!-- HTML结构 -->
<div id="list">
  <p id="java">Java</p>
  <p id="haskell">Haskell</p>
  <p id="python">Python</p>
  <p id="scheme">Scheme</p>
</div>

var
  i, c,
  list = document.getElementById('list');
for (i = 0; i < list.children.length; i++) {
  c = list.children[i]; // 拿到第i个子节点
}

// 拿到待删除节点:
var self = document.getElementById('to-be-removed');
// 拿到父节点:
var parent = self.parentElement;
// 删除:
var removed = parent.removeChild(self);
removed === self; // true

<div id="parent">
  <p>First</p>
  <p>Second</p>
</div>

var parent = document.getElementById('parent');
parent.removeChild(parent.children[0]);
parent.removeChild(parent.children[1]); // <-- 浏览器报错