Did you put the post composer into “rich text editor” mode before you pasted the content copied from the web page?
The fault still occurs.
Are you sure you followed the instructions exactly as written?
Please note that you must copy the content that is rendered from that HTML, so that the clipboard content is populated with text/html type data:
<html>
<body>
<!--StartFragment--><span style="color: rgb(0, 0, 0); font-family: "Times New Roman"; font-size: medium; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; white-space: pre; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; display: inline !important; float: none;">foo
bar
</span><!--EndFragment-->
</body>
</html>
This is not about composing your post using HTML markup.