如果你想标记文本，你需要一个支持HTML内容的元素，例如，这里有一个使用div和Selection object的版本。

$(function() {
  $("#copyAndopenOutlook").click(function(e) {
    e.preventDefault();
    // Code below

    const newLine = "\n";
    
    let emailBodyText = "";
    
    emailBodyText = emailBodyText + "<strong>This line I want bold</strong>" + newLine;
    emailBodyText = emailBodyText + "This is just another line for practice" + newLine + newLine;
    emailBodyText = emailBodyText + "<span style='color: green'>This is the last line, I want it green color</span>";
    
    
    const el = document.createElement('div');
    el.innerHTML = emailBodyText;
    el.style.position = 'absolute';
    el.style.left = '-9999px';
    document.body.appendChild(el);
    const sel = getSelection();
    sel.setBaseAndExtent(el, 0, el, el.childNodes.length);
    document.execCommand('copy');
    document.body.removeChild(el);
    
    alert("Text is copied, and can now be pasted into outlook new mail");
    
    /*
    var mail = document.createElement("a");
    mail.href = "mailto:someone@example.com?subject=Test ";
    mail.click();
    */
    
    // Code Above
    });
});

<input type="button" id="copyAndopenOutlook" value="Copy">

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

一些真正过时的浏览器使用may not support对象，但任何现代的浏览器都会。
您可能需要<br>而不是\n，或者如果"行"实际上是段落，则可能需要<p>...</p>。

或者，如果您想使用Clipboard API执行此操作：

const type = "text/html";
  const text = `<p style="color:red">this is in red</p>`;
  const blob = new Blob([text], { type });
  const data = [new ClipboardItem({ [type]: blob })];

  navigator.clipboard.write(data).then(
    () => {
      /* success */
    },
    () => {
      /* failure */
    }
  );