There is no good (online) Markdown editor that we can do spelling/grammar check the preview (online), fix the preview and sync the change back to markdown source. (spelling/grammar check doesn’t work well with markdown source, due to the extra syntax used by markdown.)
We can copy the markdown there and fix the spell and grammar error, but due to the extra syntax of markdown, Grammarly can’t find some errors which it usually can.
We can then copy the output (from Markdown enhanced preview in Atom) there and find the error and fix them in the markdown source file.