I’ve always pasted all parts of article posts into notepad, then to the forum.
When you copy from a web page, it copies as HTML into the clipboard with all that markup you don’t normally see.
Some text editors extract the textual content from the html prior to pasting which is what you really want.
Firefox has an addin called CopyText or something like that which does it for you without all the markup baggage.
You may loose line breaks and paragraph marks but...