A series of common questions people have about Slate:
One of Slate's core principles is that, unlike most other editors, it does not prescribe a specific "schema" to the content you are editing. This means that Slate's core has no concept of "block quotes" or "bold formatting".
For this most part, this leads to increased flexbility without many downsides, but there are certain cases where you have to do a bit more work. Pasting is one of those cases.
Since Slate knows nothing about your schema, it can't know how to parse pasted HTML content (or other content). So, by default whenever a user pastes content into a Slate editor, it will parse it as plain text. If you want it to be smarter about pasted content, you need to define an onPaste handler that parses the content as you wish.