To manually test this, copy and paste into the first contenteditable. The following contenteditables should reflect the pasted contents.

destination after pasting (text/html):
| <b>
|   style="caret-color: rgb(255, 0, 0); color: rgb(255, 0, 0); font-family: -webkit-standard; font-style: normal; font-variant-caps: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-tap-highlight-color: rgba(0, 0, 0, 0.4); -webkit-text-stroke-width: 0px; text-decoration: none;"
|   "LayoutTests"
|   <i>
|     "are"
|     <u>
|       "fun!"

destination after pasting (text/plain):
| "LayoutTestsarefun!"