这是不可能的textarea。您正在寻找一个内容可编辑的div，这很容易做到：

<div contenteditable="true"></div>

jsFiddle

div.editable {
    width: 300px;
    height: 200px;
    border: 1px solid #ccc;
    padding: 5px;
}

strong {
  font-weight: bold;
}


<div contenteditable="true">This is the first line.<br>
See, how the text fits here, also if<br>there is a <strong>linebreak</strong> at the end?
<br>It works nicely.
<br>
<br><span style="color: lightgreen">Great</span>.
</div>