There are a couple dangers trying to copy text from generated HTML. First, you need to make sure that all the tags are balanced, i.e. that every start tag has a matching end tag, and vice versa. It can often happen that the text at the start of your extract will be at a deeper or shallower level than the end.

Second, it is increasingly common to place as much formatting as possible in separate style sheets, and simply cross-reference the formatting you use through named references within tags. When you copy the text, all these references will be broken, unless your destination document references the same style sheet, or an equivalent one using the same names the same way. Obviously, that won't be true when pasting into a Mudcat message entry box.