When you copy and paste text from a website (or word processor etc), the browsers try to copy the formatting as well. This often causes problems as the formatting you try to paste might be wrong or not complete or out of context, therefore always paste the text without formatting.
The default Ctrl + V (or Cmd + V on a Mac) shortcut pastes text with all its original markup. But many programs include a secondary shortcut that lets you paste without formatting instead.
On Windows, you can use the combination Ctrl + Shift + V (or Option + Command + Shift + V on a Mac) to paste without formatting.
Note: after saving an items with bad html, all kinds of things can go wrong. In the worst case you need to rollback to a version before pasting the content.