It depends ?? You likely don’t want header, sidebar, and footer HTML since your WP theme already supplies those. If you paste HTML straight into the block editor, it might not be to happy about it because it’s looking for block oriented content.
You could however paste just the page’s main HTML content into a custom HTML block and all might be OK. However, any supporting resources (JS and CSS) will not be coming with it. Depending on the page’s content, it may not layout, style, or behave correctly.
Little harm in trying though. It might work. Or it might not, then you’d need to take a different approach.
