浏览器中的代码编辑器：CodeMirror
CodeMirror 是一个JavaScript 实现的可视化代码编辑器。
可以在textarea 里敲代码。
CodeMirror is a JavaScript component that provides a code editor in the browser. When a mode is available for the language you are coding in, it will color your code, and optionally help with indentation.
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