CSS ボックスに斜めの角を作成するにはどうすればよいですか?

<code class="HTML"><div class="cornered"></div>
<div class="main">Hello World</div></code>

<code class="CSS">.cornered {
  width: 160px;
  height: 0px;
  border-bottom: 40px solid red;
  border-right: 40px solid white;
}
.main {
  width: 200px;
  height: 200px;
  background-color: red;
}</code>

<code class="HTML"><div class="outer">
  <div class="cornered">It's possible to put text up here too
    but if you want it to follow the slant you have to stack
    several of these.</div>
  <div class="main">Hello hello hello hello
hello hello hello hello hello</div>
</div></code>

<code class="CSS">.outer {
    background-color: #ccffff;
    padding: 10px;
    font-size: x-small;
}
.cornered {
    width: 176px;
    height: 0px;
    border-bottom: 40px solid red;
    border-left: 40px solid transparent;
}
.main {
    width: 200px;
    height: 200px;
    background-color: red;
    padding: 0 8px;
}</code>